Personally off the Pro tees, the Srixons you are currently using are perfectly adequate. They have more than enough spin to hold the green from where you are playing your second from. I think I used the SD- tour WGT balls until I made TM.
If meter speed is an issue obviously go for a slow meter ball, ie one with more feel, like the slow meter Cally or the New Nikes or max balls but with the G20s this shouldn't be the case.
When you make Master then a ball with more spin is helpful but I feel a premium ball at this stage is a waste.