I switched from 23mm Michelin Pro 3 slicks to 25mm Vittoria Rubino non-slicks last year and the difference was night and day.
My observations:
-Michelin Pro 3 are supposed to have good puncture resistance. Fine in the summer but I found the sticky surface picked up debris and during winter this became a problem and I suffered a LOT of punctures. It was a massive problem on the Kentish Killer where the road was washed over with all sorts of sharp stuff from the melting snow and I ended up pulling out after 3 frustrating punctures in my new tyres. Vittorias are definitely better. Whichever one you choose, pick a tyre with a conventional rubber surface and it won't pick up and carry any debris. Gatorskins are normal rubber too.
-I was warned against going from slicks to non-slicks. Michelin slicks were great in the dry and fine in the wet on a good surface, but they have a habit of slipping about at slow speed on rough surfaces like pot holes and drains in the cold when it's anything more than a little bit damp. I found having a bit of light tread makes that slow speed stuff around traffic a lot safer.
-23mm to 25mm. No noticeable drop in speed, but it is noticeably smoother both because you have a bit more tyre to soak up the bumps and because you can run them a few psi lower.