I haven't had Yokohamas on either of my Boxsters but my MR2 Spyder came from the factory with Yokohama Advans. The tires were way more high performance than MR 2. They should be fine on your Box. But don't take my word for it, go out to tirerack.com and read up on them. Be sure to find the exact tire.
But if you go the Yoko route, buy 4. You don't want to mix the characteristics of 2 different brands on your car. I seem to recall seeing pics of your mountanous roads. You don't want to end up going over a guard rail because you lost control.