Well in that case, I think I'd go with your second option. I do think a 16" bar would be an improvement for limbing versus an 18". In the Scandinavian countries they run 50cc saws with 13" or 15" bars for darn near everything. If you've never watched a video of how those guys limb up conifers, you should check it out! So I guess what I'd probably do in your situation would be to swap a shorter bar onto the 261, then add a bigger saw in the 70cc range. Run that 70cc for a while, then swap to the 261. If it still doesn't feel nimble enough after running the big saw, add one of those little echos, or a 241 if you're feeling rich.
From what I've seen, those echos wake up nicely with a muffler mod and a timing advance. Personally I like a lightweight saw for limbing. I have a Dolmar 421 (stock), a Husky 350 (ported 346ne top end) and a 261 version 2 (ported). The Dolmar is the lightest, and I have a 12" bar for it, which makes it real nimble. It's got plenty of power for limbing. I don't need the power of the ported saws for limbing, but I do really like the throttle response and the chain speed. I would bet that with a little work you could turn one of thirty little echos into something that is fun to use and can be run for a couple hours without being fatiguing.