.15" is not a much of a difference in size. the tensionser should take up the slack unless it is at the max position.
what most of us do with the S2 motor is just upgrade the tensioner pulley size from 3" to 3.5". and that is for a .4"+ pulley drop.
I have to ask since you dont have it in your sig, but what are your supporting mods with the 2.7?
Also, if you do actually have slack in the belt to the point that is it slipping at WOT then you may need a new tensioner, not a smaller belt or larger idler.
1996 with 254k miles, L32 4" FWI -> ported N* -> Ported Gen V w/3.0" Pulley, Stage 3 Phenolic I/C, ZZP FMHE, 1.84 RR, Headers and 3" pipe to mufflers, F-body brakes, and lowered on Eibachs. -RIP
AMG C400 White on black -daily