Ok I'm definitely going to reduce the bumps overall. I'm going to try to reduce the bumps that have shown up in my bump maps and leave in only the panel lines. I also need to add detail to those wings and I am not entirely sure what I will add, however.
This is a personal opinion, but don't add details just for the sake of adding details. If something looks to bland, and you don't want it to look "realistic", sure, add things (I know this is what many people understand as "steampunkifying things"... I do not agree really, but then again, that is my opinion).
But if you want it to look realistic, or at least be a little bit based on reality, check out pics of real planes first. And there you will quickly find out that wings seldom have much on them besides some slight seams, and in case of old planes, maybe bolts, the odd engine if the plane has the engines mounted on the wings, and then maybe the wheels if they fold into the wing / are fixed to the wing.
Other than that, the wing is pretty much bare. BECAUSE the wings main function is to provide lift, which gets kinda hard to do if you add stuff that will disturb the airflow!
Some things that you COULD find on a realistic wing are the flaps n' stuff used for steering (you could just add something to the normal map if you do not want to make it movable), lights integrated into the front or the ailerons, ailerons (though you already have them on your wing), and the casings for the hydraulics that move the flaps (see https://en.wikipedia.org/wiki/Flap_%28aeronautics%29#/media/File:Easyjet_a319_wing_g-ezav_arp.jpg and https://upload.wikimedia.org/wikipedia/commons/2/2b/Wing.slat.600pix.jpg).
1): I think it's the intakes that really messed up the feel of the engines. Again, you can comment more on what specifically, but I feel personally that if I improve the intakes for those engines it'll look better.
Mmmm.... now I might be nitpicking, but these engines are just not aerodynamic enough for a plane. Don't get me wrong, given you are not too concerned about "realism", a non-aerodynamic engine is quite fine. Then I would just go with the rule of cool. Change the intakes, and see if it looks cooler.
Given you do care about realism, first think about making the engine round. That hard edges are bound to generate a hell of a lot of turbulences. Airplanes moved away from rectancular, boxy structures before they were even over 300 km/h fast... given you have a hell of an engine and don't care about the fuel usage, as well as wear and tear, that would still not be so much of a problem at higher speeds (might make the plane quite hard to control though).
Still, realistically, it does not make sense to make rectangular engines for a plane...
2): I was going with an A-10 feel for the Gatling guns. It might also be the out of place texturing as well for the Gatling guns? Or should they just be integrated more into the fuselage? As for the nose, I was thinking of a SciFi weapon/enhancement, which is why I didn't quite streamline it.
Again, IMO.... I would integrate those gatling guns more into the fuselage. This setup might be fine for a helicopter type of system, which doesn't need to generate lift the way a plane does (where the fuselage actually also add to the general lift, thus also needs to be streamlined), and goes at sub 500 km/h... even then, modern choppers tend to integrate their cannons into nose turrets, most probably to reduce drag and save fuel, as well as (I guess) make the radar signature smaller.
Then the whole gun assambly looks rather "short"... always remember, the mechanism that actually fires the projectiles through the rotating barrels needs to be inline with the barrel assembly.... in a cannon that is outside of the fuselage that means the full length of the cannon is visible... in your case, given the visible diameter of the barrels, that might be a short barreled gatling grenade launcher, or at best some VERY short barreled howitzer.
Something with both very low muzzle velocity (unless the round has an EXTREME amount of propellant in the casing), and a horrible accuray (which tends to be linked to the length of the barrel).
There are examples like that, airplane cannons that have explicitely been built to fit the largest possible caliber into the lightest and shortest possible weapon... see the german 30mm mk 108 (http://www.warbirdsresourcegroup.org/LRG/mk108.html)... which exhibited just the chracteristics mentioned, low muzzle velocity and accuray, with a much higher caliber and more useful HE round as its main advantage over 20mm weapons of similar weight.
I wouldn't say the texture is out of place... I do dislike the "WH40k" way of painting metal parts as "chrome metal", like it is done by many famous tabletop miniature painters. Its a try to get more color into otherwise dull military colors (which is just camo, realisticly), but to me it is wrong on so many levels. I prefer dark grey or black for the few parts that are not being painted with camo, to represent blued steel.
But these personal opinion left aside, I don't see anything particularly wrong with your choice of color.
One thing you COULD do do enhance the look of your model is to bake ambient occlusion to your model. There are many tools that can do the job for you, you basically just want to calculate an AO map or curvature map, and then blend this over the texture in Photoshop. This makes the crevices darker (and in case of the curvature map, accents the edges of the model).
IMO one of the best and cheapest ways the enhance a models look if you happen to find a tool that can bake the map automatically for you (Crazy Bump might do it, XNormals does it, a lot of other tools are available that do it. Blender might also work, don't know.)
3): Well I'm not making an RTS, but I can remove them for what I am thinking of doing.
If you plan to keep them, I would slim them down a bit. As every other part of a plane, the landing gear should be as light as possible while still being able to serve its primary objective... which is in this case to support the plane on the ground. To me, without going too much into how they could or should work, they look a little bit to "fat" for just that.