It requires more force to compress a stiffer spring so unless you up the pressure it will slow the the gun cycle rate. Anyway point being with out using a higher pressure gas a metal slide will reduce performance and using a higher pressure gas will run a greater risk of the gun breaking.
If the OP wants a metal slide Guarder is the cheapest you should go and no after market metal slides, even the high end stuff is drop in. They all require modification to fit right.