Can I run just the si impeller and 9lb pulley without water/meth injection? About how much power would that add?

How much boost are you making right now? I saw in your signature that your car's making 488whp. You're currently making that with the Sci impeller in 91?

Stock kit installed and tuned by seb at specialty z. I think I'm making 9 lbs.

You should be able to run the Si impeller and the 9lb pulley without water/meth. I ran both of those on Topgunz's gen 1 kit tuned by Seb. 488whp on the Sci and 8lb pulley that you're running right now is a great number on 91. You should do real well with the impeller and pulley upgrade

Reliability will still be good? I might email Seb and ask about it, see how much it would cost and if he has any thoughts. Kind of tempted by 2 piece lightweight rotors also.

Reliability will be fine. You may have to send the sc to vortech have the impeller installed. I don't think they'll sell just the impeller. Seb has done 3 sc upgrades on my car. I started with the Si impeller and 9lb pulley but I was on flex fuel. My car will be going to Seb soon for a FI tt install.

Belt size
 

Hey guys, I am about to install the anti slip pulley, do I need a new and longer serpentine belt?

No