I have both the Version 2 (chrome) and Version 3 (pre-ASPH) 90mm Summicrons. The V2 is a huge, heavy cannon, so I never used it very much (and it's up for sale, actually, at present). The V3 is more compact, but still chunky and heavy. It's not exactly super sharp at either f2.0 or 2.8 either.
I keep thinking I might one day trade it toward the 90mm/f2.8 Elmarit-M, which is probably as sharp or sharper at f2.8 as the Summicron is--and is a good bit lighter.
If you want to go really light, the thin 90/2.8 Tele-Elmarit is probably your best bet--I owned one for a while with a slightly dodgy front element, and I loved the size and it was decently sharp despite its condition. I also once owned the early-60s 90mm/2.8 Elmarit, and it was actually a pretty good performer, too, and not terribly heavy--optically, it's probably at least 90% as good (no pun intended) as any modern lens and is built extremely well.
I've never owned the "fat" Tele-Elmarit, but I know people who have and they were also quite pleased with its performance. Maybe not the lightest of Leica's 90s, but possibly the most compact of the faster lenses. They, and the early Elmarit I just mentioned, are probably the cheapest options for 2.8 or faster lens if you go that way.
Another possibility, which I've been a bit intrigued by, are the Voigtlanders: the Apo-Lanthar and Apo-Skopar. Anyone here used those and care to comment?