Well, it's my turn to ask for suggestions..

I think your glasspacks are on backwards, and/or restricted.There is a quickcheck for the split-overlap position, which very nearly always falls within a few degrees of TDC exhaust stroke.
You can measure the exhaust pressure with a pressure gauge. I should run less than 4psi.... 2 to 3 is better.Of course less than 1 would be excellent.This should be measured at full-load-WOT, and at or near to the engines torque peak. You can get an idea of the pressure by what's coming out of the tailpipe, but you kinda need to know what should be coming out, so that you have a basis of comparison. I just braze Nipples in the downpipes and measure it. Nice numbers to print on the work-orders. Customers like that.