Given the case looks to be that the 800s will have a far less power to weight ratio than the HSTs and the 170s, doesn't that beg the question as to why DfT/VTEC didn't go for 802s instead?
Possibly because the gradients between Perth and Inverness are a fairly minor part of VTEC - they'll be running sixty five 800/801s - possibly more if they order another half dozen to replace the remaining 225s (once Newton Aycliffe has capacity and more positive headlines can be generated from a follow on order).
The "Highland" part of the Highland Chieftain is such a small part of the overall VTEC network (and obviously the diagrams interwork with services to/from common-or-garden destinations at Kings Cross, since it's only eight hours) that it presumably isn't worth having specialist trains just to deal with a hundred miles a day.
GWR will need 802s to deal with the Devon Banks, which they run over much more often than the once-a-day Chieftain (and which won't need to interwork with Oxford/ Bristol services at Paddington), so it'll be worth having separate trains to do this.
To turn the question on its head, what would you need to do to have a small fleet of "802s" dedicated to the Chieftain? Presumably you'd need three trains, to be restricted to the diagrams that run from Neville Hill to Inverness each day (via Kings Cross), which would require separate staff training for - which would mean a lot of redundancy for the sake of a hundred difficult miles a day on a TOC that must run several thousand miles per day.