Long story short I put conical springs (supposed to be good for .650 lift)on my heads when I rebuilt the engine because my oil pump came apart.
I ported the heads, done Gpi drop in rods and pistons and cam motion stage 2 PD blower cam, katech oil pump.
My car made 487 rwhp n/a with an msd intake on it.
I put it on the Dyno again back in September last year and it made the same number.
I recently developed a skip and it ended up being a broke valve spring. They have 10k miles on them. I replaced them with cam motion .660 dual springs. I gained 18 hpw and 9 torque peaked but most importantly it gained through the whole rpm range plus carried further. I was pleasantly surprised. I think I had bad springs from the start.