As soon as one encounters drivability issues from mods, it can become very worthwhile. Some of the 94-95 guys (with the finicky puter) run them almost bone stock just to fix the 'too much timing and too little fuel' factory tune. Since the A9 series puters are more forgiving, it normally isnt necessary till a greater degree of mods are performed.
I've been running one with a 351W for quite a while. Works fine. Usually you can make some gains on the dyno with a mild N/A setup and a tweecer or chip tune.
I've been thinking about getting one but since they come with basicly nothing telling you how to work the system with the computer you are basicly left up a creek with out a paddle and really have to put alot of time and learing end to it. Alot of trial and error a friend of mine has one and he has learned alot and also gotten pissed off alot lol.
With that combo, spend the money elsewhere. There is little to gain in a properly set H/C/I setup 306.
Take the money you don't spend on the tweecer or pms, sell your windsor jrs. and your E cam.
Put the money together, get some Tw's or afrs, a custom cam, and you'll get another 40rwhp.
Istead of maybe 5 from a 500$ plus tweecer.
No point in having a 12second car with $1000 in tuning equipment, when everyone else gets there without it.
People confuse fun with usefulness with these tuners. Some people make their cars worse than better.