You need to consider a bigger blower when you have maxed out what you have and want to go faster. Are you maxing out what you have? Do you have a chassis, transmission, rear end and fuel system that can support more hp?
An F2 is not a worthwhile upgrade at all from a F1x, especially if you are spinning the F1x pretty high already. Your next logical step would be a F3-121, but that would probably add 10-15psi and it takes a pretty healthy SB to run one of those to its potential.
Hope your F1x lasts longer than my friends did 6 dyno pulls then 2 easy passes at the track then boom impeller touched down blower is junk and procharger wants $2500 to fix it and they have had it for 2 months with no return date yet.
I would buy -12 version seems like people are not breaking them. I have 370" motor and make over 30lb boost with F1x way before I shift. Thats with pretty good overlap on cam. You don't need anymore blower it's enough to go 4.60s in the 1/8 mile if car is setup. Putting an F3 on that little motor would be 50lb boost turned up