It may be of interest to some collectors due to a low number of them being produced. It probably wasn't a bad performer for the typical automobiles that were produced in the mid 80's because performance levels were down compared to the late 60's and very early 70's muscle cars. Here's an interesting site on modifying an '85 SVO: http://www.projecthp.ford23.com/svo.html
The performance of the '85 SVO was probably up to par with most of the mid 80's cars that were produced.
I have owned and driven an 85 5.0 Mustang and truthfully it was very scary to drive because of the extremely poor handling. The rear end is very loose when pushing it through corners and if you get it into an oversteer condition it didn't have enough power to feather your way out of it and you better not even think about light braking because as soon as you do the rear end of the car will come completely around . A few of the car magazines in '85 said it handled good (Ford spends a lot of money advertising it these mags) but strange as it seems Popular Mechanics had an article on the SVO's rear end set up and they really ripped its handling. IMHO all the early Mustangs didn't handle that great and I've driven a few nice ones such as my friends all original 4 speed Shelby Mustang and the same friends 69 428 Cobra Jet Mustang and these weren't great road handlers either.
It'll be a fun car but be careful pushing through the corners, you'll have an unusual car, that if I remember correctly, less than a couple thousand were made. Also let the turbo spool down a little before turning off the car that way the turbo will last a LOT longer.