Speedpro H405 pistons, what to expect for performance?

I am rebuilding a 1972 dodge 360 to put in my 72 dart. I bought a rebuild kit 1 1/2 years ago which at the time advertised it included 9.5 to 1 hypereutectic pistons (Speedpro H405 pistons), stage 2 camshaft (can't remember specs). I have a set of J heads (small valves) to use for a while to save money, Edelbrock performer intake, Edelbrock 600 cfm carb. Well I am finally close to assembling my engine (many delays, work, family, my own health) and currently read a post on another website someone has almost the same set up and is only around 8.4 compression ratio.
Should I just put a set of thin head gaskets on it and hope it will put me closer to 9 on my compression? I am limited on how much I can invest in it. It will only be street driven. Any guess on possible horsepower or if it will be sluggish?
I don't really have the extra money to just buy a new set of pistons but if this is going to make it very sluggish I may have to reconsider.