How much boost is a 69mm pulley suppose to make?
14 to 15ish... If your staying 6 rib, a 15% lower aftermarket balancer coupled w/ a 72mm might be the best way to go. Your running an Aluminator right? Every 5% = roughly 3mm
Belt slip
Some discussion about that here.
http://www.svtperformance.com/forums/showthread.php?1078777-2-3-tvs-efficiency-range&highlight=
Seems like it may have issues even with a 90mm idler on there. Might be worth getting Justin@VMP's opinion.
Corepwn how are you data logging belt slip?