The bearing itself is very smooth with no notchiness to it at all. I did think the more solid carrier would transmit more vibration but surely that would happen regardless of road speed?Maybe you could test the theory by heating the doughnut with a hair dryer?
I've done a few centre bearing carrier replacements including my own and all of them have had bearings ropey enough that I wouldn't use this fix.
I guess in a case like this it's unfortunate that the stiffer mount is more likely to transmit vibration too.
Anyway, with the plate removed and the propshaft allowed to sit in its natural position (i.e. with the rubber doughnut not pulled in any direction) there's a substantial gap between the carrier and the plate. In other words, bolting the carrier to the plate pulls the carrier downwards. I need to get a couple of longer bolts so that I can insert a spacer so I'll try that tomorrow.