Correct me if i'm wrong but doesn't the GT2540 have a 76mm compressor, aka a Garrett GT2876?
This turbo was a bit of a miss match as the tiny 53.8mm NS111 turbine wheel couldn't produce enough drive to spin the compressor effectively. The other issue the large compressor caused was choking the tiny turbine, i'm assuming that you would have a 0.86 housing so that would help. The GT2871 is a slightly better matched turbo but you won't seen much gain from switching.
If you really want to swap switch to one of the GT30 series turbos, they are a much better match for a 2.5 litre. I would suggest a GT3071 (even a GTX3071 for mid to low 300's), or as Ryan suggested a TD06-20G if you want to move away from Garrett.