I'm sure I could ride an M620 through the twisties just as fast or faster than the M900 I had or the ST2 I currently ride. The difference is that I would be doing a lot more shifting to get the speed. With the M900 or ST2, I can leave it in 3rd gear in the tight stuff, or 4th gear in the more open twisties and ignore shifting completely. I'll occasionally see the engine speed drop as low as 3000 rpms coming out of some tight corner, but the Ducati V-twin just pulls right out of there strongly anyway.
On Saturday when I was riding down the freeway to Ducati North America in Cupertino for our big group ride, I was cruising on the freeway at between 70 and 80 mph. I had to pass somebody quickly, so I dropped down a couple of gears to 4th. I realized as I was going by that I probably would have had almost the exact same acceleration if I had just left it in 6th gear. That's the biggest advantage of the larger engines.
I'm also of the opinion that you don't have to be able to get even close to the limits of a bike to enjoy what it can do. It's more important to me to enjoy the ride. I think I could be perfectly happy on an M620, but you can bet my next Monster will be either an M900 or M1000 with the dual spark engine. But I have no interest in owning an S4 or S4R. I'm completely happy with 80 hp, (+/- 5).
I feel like I'm rambling here. Get whatever bike you want and don't apologize for it. If you enjoy riding it, that's all that matters. And you get some added satisfaction if you happen to be faster than someone on a bike that should be faster according to the magazine tests.