only one brand aftermarket exhaust for ES - that is the SuperSprint exhaust.
is it loud. it can be if you make it loud... but that's not exactly legal. as is, it is quite silent.
will it help gain power... not in the way you imagine. by itself, you will only gain about 1 bhp max... you won't even feel the difference.
if you newbie, do read the many threads here that concern tuning up your car... lots of poison lists, real poison list, other poison list... etc. in the Performance section of this site.
read first, then ask questions... you'd probably ask better questions after reading all the info available...
sorry about the other USELESS posts by some bros in here - they like to Tok KOK Sing Song here (TCSS) but that's their god given rite to do so... please bear with it and scroll down what you know is just banter amongst friends... they threat this place like the msn instant message... to score post hits... and you should just ignore if you not keen to join. else, welcum to the biggest lot of TCSS people in the forum...
have fun reading