Hans,
If you do decide to try the MFlow system, I'd suggest going to Twisted Pipes. It will take a little work to match the header collector to the
X-pipe, particularly on the passenger side, as there was a slight offset with the Watsons I have.
As for dynos, I week before Bowling Green, I went to Street Stuff and dyno'd the head porting and headers I did. 385rwhp. In BG both Don and I put ours on and I dyno'd 370rwhp. SO to your point re: dynos.
My first suggestion would be to dyno the car as is at a known shop and scan it to eliminate other factors being the culprit for your power loss.
We could even look at tuning it to see if output could be optimized.
Then I'd look at addressing the exhaust if it still isn't performing to expectations. My $.02
