RE: .21tm v .18tm v .26 picco v rb 523 v Sirio S27TX .27
i have the same truck you do, or atleast it was that.
Came with the 2.5 when i got it
now i run an RB323tm which is what the 523 replaced,
It's got an unbeleavable amount of power compaired to the 2.5 shocking difference it realy is.
and tunability is second to nothing i have had, and i have had a few engines in past years.
Cant realy say much on the other engines you asked about because i dont have an experiance with them,
but i do know RB makes a great engine.