What could be causing this
 

My brother has a 1994 Z28 and it sputters (bogs) under hard throttle.

i gurantee you its a problem associated with air, fuel, or spark;)
but seriously, when was the last time it was given a tune up? have you checked fuel filter? air filter clean? give us a little more

The air filter has been recently washed, fuel filter is pretty new, plugs are platinum and about a year old.

could be the opti:(...is it throwing any codes?

Well i think it could be the opti spark have him check to see also if the car is getting hotter as it stumbles same thing happened to me and it was the opti