Since I'm here I might as well give you my rundown.
I used a stock S2000 LM, great slipstreaming battles on test course, handles like a dream on everything else.
ZZII is an awesome car for this series, provided you use decent tires, but don't use this if you want somewhat of a challenge.
FTO LM is your best bet for the raw speed on test Course, but handles like absolute crap.
RX-7 LM is the best handler, but is lacking in speed.
Just from watching the AI, it looks like using an Impreza LM is just plain suicide. I've never driven it, but I highly advise against using this.
I don't know about the Altezza LM haven't won one yet.
(Just a side note, how tuned is your Impreza?)