Uppipes are pretty much the same, I'd go with the flex personally. I have the helix up and I like it alot. I also had the helix catless dp and loved it. I was running this combo with my stock catback and it sounded a little deeper with a very noticable spool sound. The car pulled much harder with the dp, especially after 5k. Only fitment issue was an annoying rattle on the dp. The sti catback is extremely quiet.

 

just threw in up/down a few weeks ago.

install was interesting to say the least.
i ended up having a like where the midpipe and dp meet up. at first it was really loud, but now got it down to about 90% closed. i think i need another gasket.

during the install make sure to have liquid wrench, new gaskets, and some extra exhaust bolts incase you break any.

the gains from the 2 are good. although i did the stage 2 reflash at the same time with the AP, so it won't be as high for you.

don't think catback is totally needed either, it helps, but the fact that you got rid of 3 cats is what will make the big difference

 

awesome news to hear, thankx guys, i do appreciate it. thats good news to hear for sure.

 

rebbaz i kill you

ps - f the hks downpipe