For a T25 housing?

Are these limited in anyway?

Why/whats the difference with T3 an d T4 etc....

A T3 and T4 housing are in turn bigger flanges (apertures). So yes they have potential to flow more, but this is also limited by the A/R of the housing, and also the outlet size as well as the turbine size/design.

A T25 with the correct set-up can achieve 500-600bhp.

As above, T25 and T3 flanges don't have a major influence on performance up to 500-odd bhp.

However, a very big non-performance related minus against T25 flange manifolds is that they limit you to very small number of turbochargers. The most turbochagers by far have T3 flanges.

For internal wastegate t25 is fine,with external wastegate bigger turbos you need the t3 or you'll struggle upto and over 500 bhp.