Hi Pete, any of the AA series 400/500/MPR are more than capable for HFT. They're easy to work on and have many parts that can be tuned and modified. I would say that you're quite close to Kibworth...pop along and have a chat with Greg Hensman or Kieran Turner. There's a lot of good HFT experience at Kibby so you won't go far wrong...also UKNeil is a member and a very, very handy chap to know when it comes to making custom/bespoke items, damn nice chap to boot.
I have customised 2 S400 series rifles in the past for HFT. One was a s400 carbine barrel/classic cylinder, custom muzzle brake, tuned trigger and custom stock. That rifle cleared 2 national and 2 regional courses. I've currently got a s400 action, free floated carbine barrel,classic cylinder in a MPR stock. Nice rifle but i'm well known on the HFT circuit for being the Steyr man
Any questions or if you fancy a road trip to my club (Emley Moor) ,where we always have a full UKAHFT spec practice course out, then please drop me a PM.