I think the Thor figures are very reliable for comparison purposes. For a manual, adding about 10-11% to the hub figure puts you in a sensible ball park for flywheel power.
My car made 376 at the hubs (good strong BPU manual).
Baldy's car is definitely an exception - I believe you are running a few mods other than decat, exhaust and plugs aren't you Baldy? Electric fans, high boost, fmic....
The norm at Thor (hub power) for a BPU car seems to fall between 340 and 380. - Something like 375-420bhp at the flywheel.
To be hitting that 420 at the flywheel (or 380+ at the hubs from Thor) you will need to have a large proportion of the following list:
A good strong motor to start with, everything running tip top, excellent intercooling, turbos holding together at higher boost, good plugs. 1.2bar boost minimum with a full decat and good cat back exhaust, good fuel, good plugs, alternator and fan pulley/belt system in good condition, sensible induction system (stock being perfect), fuelling somewhere a little leaner than 11.5:1 (about the leanest you'd want to go on a daily driver), and a bit of luck. This is by no means an exclusive list of the necessaries for a good bpu car but hopefully shows that a decat is not the full story.
Some people will get the higher figures but inevitably they'll have a fmic, some sort of fuelling adjustment in place and a few other little tweaks (some with cams, ign timing adjustment, water injection, headwork etc and still say it's a bpu car!)