My engine was built on the stock ECU, and it ran loud and slow just like his is doing. The kid that sold it might have ran a stock ECU with all the upgrades (if he did anything at all) which would confuse the hell out of it, and cause it to go into the dreaded bog mode, which would account for it running rich and slow. I would check to see if the ECU is stock, and I would definitely have a look at the internals on the car before going back to boost, lest it be stock.