Yes you will quite easily see pre 1990 Saph Cossies going for 5k plus. There are a few stuck away in garages in fantastic condition, that only come out on the odd sunny weekend, owned by enthusiasts that can afford to drive and insure a third car only a few times a year. I remember as four star was been phased out me and a few mates were at a show. We saw a son and dad with a very nice looking standar motor, we asked them if they filled up with LRP or super unleaded, the son went off and asked the dad, and he said they only filled up the car once a year, so he wasnt sure! PMSL! Even an every day driven 3 door will go for 6k plus if its in a good nick. RS500s go for upto 15k, but then again there were only 500 made, and they have the T4 turbo, 8 injector setup (in but not wired up as standard) a daddy intercooler, hard as nails block etc....
RE your point about when you were going to buy that 400bhp one. Id be VERY cautious of owners estimates. Only proof Id take would be a RR printout, and/or the paperwork and word from a RESPECTED tuner who built it.
End of the day they are never ever going to be the most reliable car in the world. But if you bought one, and gave it a full engine rebuild and tuned it at the same time, it would be as reliable as you could possibly get it. Unfortunentally a lot of people buy one, not knowing its exact history, then just tune it. Then the engine cant take the strain and POP!
To be fair a Jap car is always going to be more reliable, its newer and its built by the Japanese! LMAO