What was the top speed before? Most will do around 100 indicated on the IMO, so if this is the case, and it s down to 85 under the same conditions, you're looking at 25%+ drop in power. And engine power in a four-stroke doesn't change drastically with minor adjustments in jetting - if you plot hp vs. a/f ratio, the curve is very flat for anything near the optimal ratio, so you're not going to get a 25% hp decrease with sub-optimal jetting unless the current jetting is WAY off. I don't think the main jet is the culprit.
I suspect something happened in the mods where you're not getting the throttle all the way open or where the slide is getting hung up. Or some other restriction on the intake side - I've seen bikes run terribly where the only problem was over-oiling of the filter or oiling the filter while it was still wet from cleaning. This just doesn't sound like jetting tweaking issue.
FWIIW, I've never "drilled my slide" and have never felt the need to. The bike runs great with tweaks in the main jet (155), raising the needle one clip, and a slight richening of the idle mixture. YMMV.