How do they compare in performance and drivability?

I am considering building a lower intake manifold for my 12A to support my Megasquirt. (Previous owners butchered the vacuum system, so why not upgrade?) I am right now considering a stock 13B TB and Upper Intake since this would allow me to retain a progressive intake system. Most of the aftermarket stuff for both 12As and 13Bs gets rid of the progressive aspect and siameses' the primaries and secondaries together. I'd like to keep drivability and mileage for the most part, and I figure the Mazda engineers designed thier rotaries with the progressive system all the way down the line for a reason.

I'm looking for testamonials about how things changed when you switched from progressive to siamesed or visa versa.