I think the P1 is a great 'table, but, having never heard your Yamaha, I can't say whether the P1 would be an upgrade. Do you have a chance to audition it before purchase?
I don't understand the reference to a plastic tonearm on the P1; it's an alloy tube with aluminum headshell. Also, the feet aren't plastic but rubber, and, as with other Rega turntables, each foot is tuned for the mass it supports at its particular location. The RB100 has shorter headshell slots than other Rega turntables, so there are many cartridges that you won't be able to attain Baerwald alignment with, but if you use Rega's own alignment scheme (which is similar to Stevenson in using less overhang and having the innermost null point closer to the spindle than Baerwald or Lofgren B), there shouldn't be a problem.
I'm not suggesting the P1 is the best 'table on the market, but it offers good sound for the price. I bought one for a friend a couple years ago, and a few of his friends have been impressed enough by hearing it in his system to get one of their own. As always, it's best if you can hear it with your own ears before making a decision.