First RPMs are no where near the same. IIRC, the no-load WOT rpm on a MS170 is around 8500 rpm, where as the no-load RPM on stock 261 is 14,000 rpm. Also, RPM is not the same thing as chain speed. I suspect that the 30cc power heads run a smaller sprocket in order to generate enough torque. That means that even when both saws are at the same RPM, the 30cc saw is going to have a noticeably slower chain speed.
My general advice on saws is that if you only generally turn the saw long enough to burn about 1 tank of fuel, almost any saw is fine. On the flip side, if you're normal working the saw all day long when you run it, get a pro saw sized for the work you're doing. Working the saw all day long doesn't mean that you're going through several gallons of fuel either. You may only burn 2 or 3 tanks depending on what you're actually doing. Regardless of how much fuel you burn, you have the saw in your hands for several hours. If that's the case you want to minimize the weight without sacrificing much speed in the cut. On an 8" log, a 261 may only be a few seconds faster. While that few seconds doesn't add up to more than a couple of minutes over the course of the day, you'll get a LOT more work done with the faster saw. The more you have to stand around and wait for a tool to work, the slower you're going to be at everything else as well. There's nothing wrong with that if you're just cutting an occasional rick of firewood. When you need to fell, buck, and stack 3 or 4 cords of hardwood in a day, standing around and waiting on something isn't really an option, and lugging around an extra 2 lbs of weight for no additional output will take a serious toll on your body by the end of the day. Will a 30cc saw cut an 8" log? Yup, without a problem. Will a 30cc saw buck 4 cords worth of those logs in a day, not a chance. Whether or not a 30cc saw would be able to cut enough firewood to heat a house depends on how much you need to cut and how much time you have to do it. I need to get the bulk of mine done over the course of 2 or 3 Saturdays, so those are 10 to 12 hr days for me, and a 30cc saw isn't very useful to me on those days. Cutting on a steep incline is another story though.