Possibly a dumb question?
 

But what does clipping the turbine wheels accomplish? Less spool up?

Hmm... I thought about it and clipping the turbines would actually probably add to the amount of time it would take for them to spool. But wouldn't they produce more power then since you'd be able to get more exhaust through? I'm talking about the stock twins, but figured you guys would know more about the workings of the turbos themselves. Anybody wanna chime in here?

your on the right track.... youll get more flow out of the turbine wheel
a slower spool time but less rotating mass to help offset that.

zbrown is correct. The blades are typically clipped at different angles depending on what type of "map" is desired. A steep angle needs slightly more time to spool up but allows higher exhaust flow at high turbine speeds/ boost levels. The opposite applies for shallow clip angles. Keep in mind there actually isn't all that much material removed, a little goes a long way.

As mentioned a clipped wheel will flow more at the expense of turbine efficiency- which usually means slower spool despite the fact the wheel is a bit lighter.

In some cases a clip may actually improve spool.

If you have an undersized exhaust housing (AR or frame size) on a high overlap motor it will cause excessive backpressure while spooling and lots of exhaust gas reversion from your overlap will occur hurting your motors efficiency.

If for some reason you do not want to put the proper size exhaust housing on you may be able to clip the turbine wheel so it flows more. This can decrease the backpressure while spooling causing less reversion and better motor effciency so the turbo actually spools faster.

I found this on my stock hybrid S5 turbo w/ "P" trim exhaust wheel. It spools better w/ a 15 deg clip than a 7 deg clip.