Hi, we run a DEP pipe on our 02 YZ125 and it does help top-end revs but doesn't do anything at low rpm. You can get pipes to increase bottom end power or top end power but you can't get one to make it better across the range - except for the FMF SST pipe - which FMF say gives a 20% performance across the range - but if that were true there isn't a rider out there who would run anything else!
Most European pipes concentrate on providing additional top-end power only and as you ride 125s on the pipe nearly all the time it's probably the best option. If you want to do enduros or trail riding and need some more bottom end the best option is a flywheel weight to generate some inertia.
Pro-riders may have different pipes for different circuits - but they have time to practice with the different options and make a considered choice. Turn up at any clubman's UK moto x meeting and all you get is about two or three laps - barely enough time to sort out jetting and tyre pressures, let alone trying three or four exhaust combinations!
It won't really matter whether you've got a DEP, HGS, FMF, Pro Circuit or whatever, it will probably give more top end than the standard pipe and without a dyno test you'd be hard pressed to tell the difference between them.
If you go practicing for a day and there are other riders there on similar bikes who might like to swap pipes around for a few laps, you might find one make is slightly better suited to your needs than another. If you have to buy them to try them then you'll be spending £1,000 or more for very little gain.
MC