You should go with the RS Guitarworks 500k pot upgrade for volume and tone. I like a .015 cap in the neck and a .022 in the bridge with 50's style wiring. As far as cap type goes, the guys with dogs' ears who like "vintage tone" prefer paper in oil caps like Jensen which cost about $15 a cap. I have $2 orange drop caps and they work just fine. Check out rsguitarworks.net
It's a great upgrade, and definately should be done first before changing pickups. I have Fralin's in my SG Classic right now and I'm not in love with them-- I'm thinking about going with SD Antiquitys.
You may want to try the cap job before you change the pickups. If you like the way your current p90's growl & bark, you might not get that at all from the Lollars. I have a pair of his pickups in one of my guitars & I don't like the bridge p/u at all. Very polite & bright. The neck one sounds good though.
Just went thru this a few months ago with a Classic.
First swapped the stock p-90's for Lollars then did the RS thing with the 500k pots.
Overall I'm happy...thought the stock p-90s were ok but the Lollars have more definition.They seem brighter overall and polite(HiFi?) is probably as good as word as any to describe them.More definition less crunch.I get most of my dirt from pedals so I'm not really looking for an aggressive pickup.At times I miss some of the p-90 raunch but I was really wanting a "big" Single ciol sound anyway.
I would try the RS kit first.
IIRC my stock Gibby pots read like 280k so the pots will brighten things up a bit and may get more what you want.