I don't think I have ever had a TF trans that kicked down to 1st from 2nd unless I was going super slow.
If I did I sure don't remember it.
Also, it seems to me they all shifted too early in stock form, and yours sounds about the usual.
One thing to note is that the TP can be really sensitive to small changes, so it does take some playing with.
I installed the "first an any speed" option of my TF2 shift kit just because of the trans not kicking down into low.
This part of the shift kit allows a manual downshift to low at ANY speed, which a stock trans won't do once a specific speed is reached.
I realize this probably doesn't answer all your concerns, but it doesn't sound like you have anything major or particularly out of the ordinary going on.