What blower do you have?
A member here called "Sideways" did all of the research and spent months trying to get his Vortech S/C to the highest possible power levels.
He ended up getting spacers made up and custom 8-rib pulleys from Lee at ASP racing in Texas because the biggest issue was belt slip with the original 6-rib pulleys.
You can probably reach around 540hp @ 24psi on a 4" 8-rib pulley.

I copied his idea and the local Pro Shop here in Australia worked out what pulleys I needed to order from ASP to max out my new Vortech V2 blower.
I got the 7.5" crank pulley pulley from ASP and a number of Supercharger pulleys for various power levels (see below)
4.00” is as small as I would go if you are planning to max the blower out. All of the pulleys had spacers built in so it would allow for the extra room for the 8-ribs set up.
Also a flat water pump pulley is required, ASP provided me with this too.

4.7” @ 9000rpm = 51,702 Blower rpm
4.5” @ 9000rpm = 53,999 Blower rpm
4.3” @ 9000rpm = 56,511 Blower rpm
4.0” @ 9000rpm = 60,750 Blower rpm (Maximum)
3.8” @ 9000rpm = 63,947 Blower rpm (OVER SPEED)

In addition to the above I got:

Walbro 450 (stainless steel fuel pump, e85 friendly)
ID1300's injectors
Fuel Pressure regulator (can't remember the name)
Haltech Platinum Pro ECU
Haltech Flex Fuel set-up (so I can simply fill up with e85 and the tune automatically changes and allows for 100hp more etc..)
AP2 springs & retainers for the Head.

It's a LOT of stuffing around.
I ended up selling it all and buying a Sheepey Vband turbo kit with a Garrett GTX3076R and I absolutely love my car.

 

Is there a reason to upgrade to the AP2 springs too and not just the retainers? This is probably the first time I've seen the springs being mentioned. Just curious.

 

[left]

 

For your goals mate you are probably best off maxing out the 6-Rib set up, the 8-rip was a huge pain in the ass. If you can get a grooved Richard Racing style blower pulley it will not slip and therefore a smaller one might net you around 400hp or 13-15psi etc...
If you can't find a RR pulley for sale then I'd suggest to get one made up.

 

I second Mark's sentiments. ASP is local to me, and I also have an 8 rib setup. Mine is a bit different from sideways. I went with different offsets for less hassle, but as a result the water pulley requires a custom offset that has VERY close tolerances. Meaning you only have a couple mm or so between it and the crank pulley and space for the actual belt. It works fine, but it does make OEM belt changes a little more cumbersome. You also have to machine the standoffs for the pulleys on the blower bracket. The 8 ribs pulleys are thicker and you have to account for the new offsets. The good thing is if you want to do this, all you have to do is contact Lee at ASP and ask to duplicate my setup.

 

^ Awesome info there, well done mate.

 

Are these figures with the stock or larger crank pulley? When the blower reaches max RPM surely its making its rated max boost at that point, correct me if I'm wrong?

 

No need to change springs.