There have been many posts claiming the Tivo is mostly CPU limited. I would think this is still the case even with the increased speed of the Roamios. An SSD may shave a few secs off of some I/O operations.
Did you time boot up speed? The largest time savings, if any, would likely happen there.
Roamio Plus (3TB)
THD 2TB x 2
Humax S2 300GB (retired)