Man,
Those numbers are not anything to sneeze at. It shows me that Thumper's heads do run neck and neck with stock GT40 heads...you're pushing, if you figure a drivetrain loss of 15%, you're looking at 299 FWHP and 290 FWTQ...that's not all that bad.
I noticed something in your sig, you don't list any headers...are you using stock headers? My advice, find yourself some 1 5/8" LT headers, and OR H or X pipe. The other area that you might look at would be Underdrive pulleys and an Electric Fan. You probably won't hit the 300RWHP mark wtih that, but you should get a nice kick in the pants, and maybe get you to the 270-280 RWHP mark. If you need more power, you might look at a different camshaft...I have no gripes with the E cam, it's not bad, but there are better one's available. SO...UD Pulleys, Electric Fan, Uncork the Exhaust, and pick a more efficient camshaft, and you should be better off.
One other thing I want to point out, from what I have read (both on his site and on the board), Thumper's heads are not designed for a high peak number, but to make a lot of torque in a useable RPM band, that is better than stock. Thumper looks for quick throttle response and neck snapping torque...not a high RPM tempermental narrow powerbanded PITA. I would love to see the graph, and see what the torque/hp curve looks like....my bet is you get a nice broad curve that is all useable and probably very fun to drive. It sounds to me like Thumper's heads are delivering as advertised....I just hope you did not have unrealistic expectations.
Sir Hacksalot