your most available option is 3.9, and a bit harder to get is 3.7

there is calculators on the net you can enter your speed/tyre size/diff ratio/gear ratio and it will tell you the rpm, then you can enter with a different diff ratio and see the new rpm. best to use that maths than someones "experience" anyway

you will obviously be cutting acceleration, and it is noticable, but if you have a nice wide power range with your turbo it may not be such a problem