Hey Wyvern Claw,
I got myself 2xTM sig 552, 1xJing Gong Sig 552 and a ICS Sig 551. So I'm a bit familiar with the gun, not an expert mind you but I know a bit.
Regarding the battery. One, you can put up to a lipo in it to shoot faster and last longer or you can attach an outside PEQ box to shoot faster and longer.
If you upgrade your spring you run the risk of wearing down your TM gears a lot faster if not strip them altogether - depending on the fps your after. Around 350-360 you should re-shim and put in metal bushings for sure.
Remember its a CQB design so getting close is the key with these guns. They are very accurate stock guns. I would resist the urge to do any upgrading till something breaks in a TM gearbox. I have one Tm shooting a 400 fps and the other TM at 300 fps and to tell you the truth, I prefer the 300 fps. Seems to be more accurate, quieter and higher rate of fire.
I got a 7.4V lipo inside it.
That's my two bits.