Bryston's online manual for the 3/4/5/7/8B STs includes drawings of the rear of the units, which indicate that the 5/8B STs have the power on/off function, whereas the 3/4/7B STs don't, and the 8B ST THX does.
The manual also refers to this function as being optional, so I suspect you could order it on a non-THX ST version.
A Bryston brochure also says "All ST amplifiers are available in THX version, include remote power on/off function."
The implication of this to me, as someone searching eBay, Audiogon, etc. for a Bryston power amp, is to make sure I ask the seller of non-THX versions if this feature is present or not. I have a Bryston pre-amp with it, so I would definitely want it.
The upshot is it seems I don't have to be concerned that the THX versions contain any design trade offs to comply with the THX standard that would effect my listening experience in 2 ch listening.
BTW I recall a posting on one of the forums where Tanner (Bryston VP) was commenting on a similar issue: the difference between base and pro versions. He said (in my own words from memory, since I can't find that posting right now) there was no fundamental difference, other than the pro version having a feature to attenuate the incoming signals to facilitate matching amp gains. He cautioned those who prefer a purer signal path (w/o attenuation electronics/switches) should stay with the non-pro version.