I got a great deal on a 2011 MS660R 3/4 wrap HO oiler from a logger who used it as his #1 saw for a year. He replaced it with another MS660. FWIW
I have not done any port work to it yet. I put a bigger jet in it and that made a difference, dual ported the muffler, that helped a bit.
It has PLENTY of power and I find myself using the felling dogs all the time because it pulls so darn hard. It runs a 28" bar with 8 pin rim most of the time. I have a 36" bar and 7 pin when stuff gets a little bigger. I've never had any trouble with it struggling with anything. Lean on it, let it self feed, it don't care. I've put a boat-load of hours on it since I've had it with ZERO issues.
I've seen guys with hot rod 70cc saws that cut like a hot knife through butter until the big bars come out. Then the less than 1lb weight difference is more than made up for by the grunt the MS660 has.
I can't personally find anything bad to say about my MS660. Only thing that might be considered a down side is 23 or 24 tanks of fuel in a full day of use. But DAYUM a lot of work gets DONE.