Does being an ultrapeer for other users slow down my download speed?

maybe, but probably not. Acting as an Ultrapeer uses about 4 or 5 KB/sec of your download speed. If you are maxing out your download bandwidth (doubtful), then being an UP may slow you down slightly.

However, LW does work in some advantages to being an UP, so I'd guess you are still better off somehow.

Of course, the best test is to turn UP cpapbilities on and off in the options, and try to see if it makes a difference on your set-up. You'll need to run quite a few tests with UP on and off, because download speed depends mostly on the ability of the sources to serve the file(s) you want.