I will try to add some tech to this thread, Its a balance when it comes to sizing the turbo and turbine ar size. Its easy to get extreme ends of the spectrum. IE you can go for fast spool and transient response with more low end torque but you give up rpm range up top and the ability to hold power up there. Or you sacrifice that low end torque, response and spool time, but make more power up top that holds to a high rpm.
And its also related to the power and boost level. More specifically pressure ratio, which is a number you can get from a turbo calculator that is a function of 4 things. Engine displacement, volumetric efficiency of the motor, engine speed(rpm) and boost pressure.
This can be tricky to do and may take trial and error and does have other factor such as kit design, vehicle weight, trans, gearing, tire height etc.
You may get a turbo that has a good balance of spool and top end power but it will go inefficient as the pressure ratio factors change. Ie it may be super responsive and hold power to 7k rpms at say 12psi but then you up the boost a few psi and now it can only hold power to 6500 rpm. Few more psi and now power is dropping off hard at 6krpms.
Going up in turbo size turbine/ar/compressor may now keep power to that higher rpm at the higher pressure ratio but you will sacrifice response and spool time. And peak torque and power will shift higher in the rpm range.
So its a balance when you want to have your cake and eat it to. Selecting a turbo that's a balance of both to fit your setup is the easiest way to try and do it. So in short if you want to maximize lowend you have to get a turbo that is just big enough to support your max power and rpm goal and no larger. As you go larger on the turbo you move the surge line and efficiency of the turbo though. Which simply could mean you need more boost than you did to get to the same pressure ratio or effect how well the turbo performs at lower boost pressures.
You mentioned you want 650-850hp at 7000-7500rpm that's a wide range. You would have to build for that top number. You have to get a big enough turbo to make 850 at 7500 rpms, just keep it mind it wont have the same low end response of a turbo that would do 650hp at 7k rpms when sized for that.
Things that help widen your room for error or ability to get a balanced setup. Billet extended tip compressor wheel designs. They have lower inertia and flow more air on smaller wheels ie response of a small wheel with flow of a larger wheel. They also flow better at higher pressure ratios. They achieve this with smaller hubs and wider vanes with a revised wheel design so even though inducer diameter is the same you get more response and flow.
From there when having to go larger on the turbine size and turbine housing ar to support the topend power and rpm you want, you can get more lowend response by moving the turbine closer to the engine. When its 1-2 ft from the head vice 6-9ft away with a front mount you will load the turbine faster as there is less exhaust volume to fill pre turbine. Trans type plays a roll an auto with a stall will load better than a manual ie you can get away with a larger turbo on an automatic car. A heavier flywheel and driveshaft will load better as well. Rearend gear and tire height also can be played with to load better.
So essentially you have to really size for the power and rpm you want and you will get what you get when it comes to spool and transient response outside of playing with the factors I stated above.
Some data for you that I have gathered, a 6765 turbo with .96ar or .81ar housing will hold power to 7krpms at 650rwhp roughly 14-18psi range. It can be pushed to 18-20psi 700-725rwhp but power will not hold to 7krpms. As it does at the lower power and boost level. Some more rough average data 6871 750-800, 7275 850-900hp, 7679 900-1000hp. These size will maintain spool at these power levels and as always can be pushed but you start lowering flow up top at high rpm.
ok i will buying Precision 7275 i found in ebay :
What do you think about this link that ?
http://www.ebay.com/itm/PTE-7275-tu...-74mm-7575-t76-t72-t78-t88-7675-/281259373178
Last thought, is what are your intake manifold, heads, cams, valve spring setup. 7000-7500rpm is a bit of stretch for a 3v boosted setup with out mods here. Most will be in the 6500-7000rpm range here otherwise your exceeding the ve of the 3v motor setup, and adding boost wont add power where the engine cant move air anyway.