I own the 42, 52 and 62 machines at my location and of course all these machine only work as well as they are calibrated. I have used and worked on them from the mid 80's so I have a very good knowledge of their capabilities.
I had a Tascam 42B in my shop do a sweep of 25Hz to 50KHz inside of plus or minus 2 dB at 0 VU with a speed of 15 IPS and using SM900 tape. I know that does not make any difference but just goes to show what you can do with these and the right tape. These heads were not new either.
As far as Tape Project tapes. The deck can work with IEC or NAB depending on what alignment tape you set it up with. Between the two the difference is at most 3 dB so I would not worry about that as most tone controls are more than sufficient for the correction were you to even detect it. There are jumpers or switches on the cards for you to make the change.
If there are persons out there that do not like theirs then that is no problem as I get requests for these units almost once a week. Sell it to me. for those who do not know me, I worked for Teac at Chicago Factory Service that was located in Arlington Heights.
I still service decks all the time.