I can politely disagree. x86 is only top notch due to laziness, spending by Intel, and people not knowing any better. But even then the fastest super computers are intermixed with ARM and custom silicon or are PowerPC based. x86 is a terrible architecture for computing and if you want to know what x86 is best at, it's funneling data to custom computer cards anymore. It's so bad that Intel talks about it's non-x86 compute cards as their high performance computing processor.
As far as mainframes, they scale out better than your average cluster. The only downside to mainframes (besides listening to Andy rave about AIX) is really only the cost structure. If you care about the overall cost of your data center, then maxing out mainframes will his a value marker of cost, cooling, performance, and uptime you would be hard pressed to hit with any cluster. If you're some scared IT manager, their support contracts on that hardware are amazing.
I don't know if the talk was recorded or not, but the talk at PLUG North a few years ago on SystemZ Linux was AMAZING and highlighted why mainframes are still VERY relevant. Basically choose between mainframes and arm based solutions (which virtually don't exist) to get the performance, price, and fit in the thermal profile the data center where x86 can't compete if you're getting that serious.
-Will