5.5T should use 8.5 gear ratio.
Your setting is fine now.
If change a bigger pinion, the top speed will higher but weak torsion.
Also the motor will get more heat than before.
I can't remember how much is it exactly, It's around $1100 - $1200, you probable could pay less than me cause I won't waste my time to shop around and search for the cheapest parts.