How much HP do you want, and how much money to you have?
Seems like everyone has some boost drop. It seems no one knows for sure when/if belt slip occurs.
My personal opinion is, without heroic measure a 2.93 (what I have) does have some slip. At least I have more boost drop than I can attribute to efficiency issues.
I think the smallest upper that won't slip is a 3.1.
I think the 3.1 , while it will eliminate belt slip, does not turn the blower quite fast enough. I think a 3.1 upper with a 2 lb lower would be ideal. If you go that route, and have the interchangeable lower, it wouldn't be that expensive to buy the 4# and 6 # rings to see if they do perform any better.
I have a webpage that shows why my personal opinion is that more than the 2# lower with the 3.1 upper is counterproductive.