The only speed up kit Deere makes is for 9600 beater drive and it's actually a slow down kit with the pullies reversed. It gives roughly a 30% increase. Last one I bought was $402 canadian dollars. Should be less in USD. Part # BH78623. It's a whole goods kit which means it takes 1-2 weeks for a dealer to get it.
The PF kit is $492 USD and gives a 60% speed up.
The PF STS rear beater speed up kit is $269.43 USD