That is not true. I guess you are referring to the infamous OCZ case. The performance is the same as long as the amounts of NANDs is equal. In the OCZ case, OCZ used higher capacity NANDs in some lower-end models, meaning that less channels were populated. For example in the 64GB model, it used to use 16x4GB 32nm NANDs but when OCZ switched to 25nm, they doubled the density and used only eight 8GB NANDs. When less channels are populated, the performance degrades. However, being 25nm or 34nm does not affect this, at least not significantly.
It IS true. OWC's change to 2?nm has also failed to reach the advertised speeds.