Changing to a colder thermostat will do nothing until you change the PCM.
Right now i have a 3.4 , with a FWI (fender well intake) and colder plugs (TR55),and a high flow cat.
And i run the stock belt. , no KR.(check your boost gage to see if your boost go higher, you will know if your belt slip, it should go near the end of the yellow part)
The best is to have the car scan after the install to see your amount of KR (knock retard)
due to a smaller pulley, but you can be fine with a GOOD intake and colder plugs.
(You need to protect your filter from heat, fender well intake are the best.IMO)
A good heat shield can do the job too.
Dont forget to reset your PCM , (unplug your battery while you change the pulley and replug after, it should reset it)
Also, get the car a bit warm to help remove the pulley. And let it cold down to put the new one, it will be easier.
I went from 15.2@92 to 14.9@94 with the 3.4 pulley, and 14.6@96 with the high flow cat.
And like dbeast420 said... youll love the torque
(save money for new tires)
Let us know how it run.
Once youll begin, it will never end