Yes, can't claim 250bhp quite yet
Somewhere in that ballpark is my goal, but I'm going to get the turbo motor bedded in first before I start getting all excited about additional power. I'm planning to keep it down to that sort of level to preserve the drivetrain...har har!
Rally cars appear to be very hard on their drivetrains. John Lane (500+bhp turbo PRV V6 262 rally car) is constantly telling people on Tbricks to throw away their 'jelly' M46s, since he made a mess of one with just 140bhp. Another guy with a street car is putting just shy of 400lbft of torque through his, and has been for years...and he doesn't baby it!
Two 300s which definitely are making relatively big power are Huskyracer's 340 Turbo (~210bhp on a trusted dyno), and Kev Strong's 360 turbo (claimed 315bhp). Kev's car is running a B230ET bored to 2.5 with extensive headwork, plus balanced to rev out to ~7500rpm...and fuel system add-ons to match. Whether or not that's a formula for 315bhp is a matter for some debate, but especially with that extra RPM, it's believable.
Given that the M4x series in front-mount guise are known to reliably hold at least 250lbft in road use, there has to be something in the 'usage' when rally cars seem to commonly trash their boxes with only around 2/3rds of that torque. We've seen the same pattern on Tbricks, although us UK guys have always put that down to the yanks not being able to drive properly
cheers
James