mglincoln --
I look forward to hearing about your new adventures with turbocharging an I-6.
I really like the T5 tranny in my mustang. The car came stock with a non-syncro 3-speed, so anything would have been an improvement, but the T5's range really wakes it up. I have always preferred manuals over automatics, so that'd be the way I'll always suggest for anyone to go, too. I don't know enough about the automatics to comment.
As far as the performance I get from mine, I've never taken it to a dyno, so I can't say for sure. Calculations, from McInnes' book, suggest my stock, carbureted 250 with 4# boost should be hitting about 197 HP. Ak Miller, the highly recognized guru of I6 turbocharging who designed and installed it, said a set-up like mine should hit 195hp, and in a light Falcon/Mustang body ought to do a 95-100 mph quarter mile. I still have the fairly weak, stock 6-cylinder differential in it, so have never tried to do any serious drag racing -- I'm not interested enough in 1/4 mile speeds to grind up the diff and leave bouncing behind me.
Before I left California in 1989 I took it out on the rural roads in the valley -- similar to the flat, laser straight, long, long roads in eastern CO -- and floored it. The old Falcon-style speedo pegged at 120mph, the tach was at 3300 rpm, and my seat-of-the-pants dyno suggested it was just starting to breathe nicely. With the T5 and 3.2 diff, it has a _theoretical_ top speed at 4400 rpm of just about 160 mph. I doubt it has the HP to actually do that, but I do know that it still had plenty of 'oomph' (a technical turbocharging term...) at 120mph. However, at that point my brain was screaming to remind me that most of the Mustang was bone stock, nearly 25 years old, and was _never_ designed to go so fast -- so I didn't push it any further.
I average about 22 mpg on 92-93 octane. Ak felt it would run on 91 octane, but I was more comfortable with premium. The best mileage I can get is around 27 mpg on a mostly freeway trip. The worst I got was a hard driving 'on- the- turbo- as- much- as- I- could- without- careening- off- the- road' just under 17 mpg on a trip from the San Joaquin valley over the Coastal range to the Central Coast. Legal highway speeds (55-65 mph) with the T5 have the engine running in the 1500-1800 rpm range in 5th gear, and a quick downshift to 3rd gear will jump it to around 3200-3600 rpm and allow it to accelerate quickly.
It has run like a champ, but after about 15 years with this set-up, I am itching to upgrade again. Since it is an Ak Miller system, I don't want to hack it apart -- it may be more valuable in an "Antiques Roadshow" sort of way than anything I could re-fabricate it into. Anyway, I would really like to go for EFI, intercool it, and raise the boost. That'll mean some large investments in body stiffness, suspension, and drive train (probably bite the bullet and go to an 8" diff and 5-bolt wheels all around). It needs body work right now (after 40 years, rust and leaks are appearing), so it has been up on blocks in my garage for far longer than I care to say.
Again, good luck. Take your time, and keep your main goals in mind. You've waited this long to upgrade, so don't be in a hurry. Speed costs a lot; mistakes cost much more. Most of all, have fun.
--- Barrett