I sort of agree with you - if you need twitter type scale, you will need to rewrite.

Most frameworks are bloated (not setup to REALLY scale) out of the gate.

CakePHP is very flexible, and if used correctly can scale for 99% of workloads. Especially if you leverage some of the php5 only nuances (cake 1.3 has been developed to support php4 and php5).

CakePlate can steer you in the right direction - but is not an end all. It helps turn your idea into reality quicker. If your idea twitter scales, then you have cash to throw at the (good) problem.

If you give Cake an honest shot, I'm confident you will like it.