if your only getting 1.5 deg of knock on a cold night from a caned tune then there is probably alot left in it. im sure fueling and timing could be tweeked some to get a bit more power.
if you are going to get it dyno tuned or custom tuned i would throw the 3.6 on there and let them tune it in for that. if they cant get some decent timing out of it just put the 3.8 back on.
thats the way i would get pullies tho, by the .2 .3 is a little large of a jump and .1 is not that much of a difference.