I'd recommend just getting an AEG that shoots sub 400 and tuning it up first before dropping in a beefier spring. You'll be able to use it with full auto in the meantime without being at the disadvantage of a sub par DMR on semi. Get your compression system and barrel assembly sorted out first. Once your FPS is +/- say 3FPS and you're happy with the precision, drop in a heavier spring to get you as close to 450fps as possible and move up to quality .36g bbs. Improving trigger response would also be something to work on, though simply getting something like a ZCI or SHS high torque motor may solve that problem well 'enough'.
There's a ton of free/very cheap mods that can be done to improve the performance of a stock gun. I call it 'setting a good baseline'.
Basics (Do these first before doing any mods):
- Run a decent 2s or 3s LiPo
- Use Deans plugs
- Use quality .28g+ BBs
- Keep your barrel clean
If you gun doesn’t have them:
- Metal spring guide
- Metal bushings
Once the above is accomplished, move on to the little mods.
Inside Gearbox Mods:
- Re-shim your gears
- Re-grease gears and compression system
- Correct AOE
- Polish cylinder
- New piston head o-ring
- Teflon cylinder head
- Radius cylinder cutouts
Outside Gearbox Mods:
- Shim hopup arm
- Shim inner barrel and assembly
- Flat hop
- Teflon/Floss/Permatex bucking
From there, upgrade to accomplish the previously mentioned goals. You know, I've considered modernizing the 'Holy Grail' thread, but there is a ton of that information out there so I just assume it wouldn't be that useful. But maybe I'll do a write up on my version of that thread once I buy a used AK and I'll do a comparison from stock to tuned.
EDIT Again: After just now reviewing that thread, it seems pretty k. Maybe I won't.