For the UK the max spring strengths you need for full-auto AEG's, with stock 6.08mm diameter inner barrels, are M105 or M110, depending on how long the barrel is and whether you're trying to get under 350 or 370 FPS. You need to consider also that a TBB increases the FPS you get from your spring/cylinder set up - for medium-long length barrels such as 455+mm you can expect to gain up to 30FPS by going from 6.08mm to 6.03mm and as much as another 30 by going to 6.01mm, which is important because the less powerful your spring needs to be, the faster your motor can pull it.
The conventional wisdom is that you need to use a high torque motor with low ratio gears, and with M120+ springs that makes total sense because lower ratios are harder for the motor to pull as well as the more powerful spring, but with an M105 spring a motor like an ICS Turbo 3000 will cycle 13:1 gears at over 25 rps on an 8.4V NiMh battery. Generally speaking higher torque motors will spin more slowly, so if you don't actually need that amount of torque to pull your spring, you're losing cycle speed for no good reason, not to mention having to pay through the nose for it.
So let's say we're talking about an M4 with a 363x6.01mm TBB: to get that firing under 350FPS needs an M90 spring and even a TM EG1000 can cycle 13:1 gears at over 20rps in that set up with an 8.4V battery and an ICS Turbo 3000 will get 27rps.
But there is more to it - piston weight, bearings/bushings, shimming, type of grease, etc. and of course battery voltage, capacity, and delivery rate ( C ). When you consider how fast an 11.1V LiPo can spin any motor 13:1 gears start to look like unnecessary strain on the whole system, but of course that does require an Active Breaking MOSFET, which will also improve trigger response, and if you get a LiPo with excess capacity, say 1500mAh at 25C continuous 50C burst, combined with the MOSFET that will give even faster trigger response.
The ultimate solution however has to be the BTC Chimera MOSFET and Control Unit - pre-cocking gives the same trigger response as a springer and even if you want your GB cycling at very high speed, say 35rps, the control unit can bring the ROF down to a reasonable figure for use with midcap mags without slowing the cycle speed, but by spacing out those cycles into, say 20, discrete pre-cocked shots in a second with 21ms gaps between them - apparently it sounds much more realistic too.
Yeah, Siegetek gears are supposed to be very good, but when you consider how expensive they are you have to wonder just how much better than SHS can they be, when SHS gears are already the dogs' bollocks, and is that actually worth so much? I spose it depends how much money you have to spend...
