hmm,

ok you dont say what 850 you have....but i have got news for you they are quicker than the comparative 700/900 series especially the turbo's ..simply more power/less weight....

secondly theres a lot being talked about better fuels -- well lets examine that thought a second...the critical thing here for efficiency is the fuel/air mixture (if perfect about 1:14 i believe)....and there are some very clever things the engine measures to adjust that (i.e. mass of air going in, fuel going in and the gas comming out)..

now the mass of the air is used because cold dense air has more effective 'air' to add to the mixture for the same volume and has more mass...hence mass is used because thats what we are interested in...

fuel measued is (as i understand it) adjusted effectively by the time of the injector pulse i.e. the length of time the injector is held open (longer = more fuel) and from some basic maths a injector of x diameter held open for y time with fuel at w presure gives a volume of fuel...

the gas analasis is then used to 'fine tune' the mix...which is why when you make a radical change to the engine (i.e. take your foot off or better still plant it in a turbo) you can get a 'cough' of smoke as unburnt fuel flies outa the back before the gas sensor has had time to 'tune' the engine.

now back to your issues:-

1. 19-20 mpg for a turbo is respectable mpg, it would be high in an NA but not in a turbo- especially if you plant your foot a bit.

2. from the above as long as the octane of the fuel is within tolarance it doesnt make a blind bit of difference what you run - the managment system will adjust anyway on the gas readings...and turbo's are notorious in running fairly rich (i.e. safe) anyway...but it wont hurt to run higher octane...

3. The best fuel to run it on is whatever the engine management system was designed to run on..because remember the only time the gas anlysis isnt effective i.e. radical change in engine behaviour, it guesses based on amount of fuel verses air its swallowing and it guesses based on time injectors are open...so the only time thats going to be accurate is when your running the fuel they designed the system for...

4. if you want more power run a leaner engine....i.e. (within reason) leaner engines generate more power...but they run hotter and are less reliable...at the extreme pistons melt....

5. Your 850 engine is a generation later, will have much better engine management and will be more powerfull...but lets see which engines last longer....

6. I would check your air ways tubes etc...pull the S^&t piece of foam out of the top of the airbox and genrally tune the car *oh and make sure you have the right air filter - turbo's use a different one)..but purely because if a 700/900 turbo doesnt go fast enough to scare you then either (a) it needs sorting or (b) you need your license taking off you...