I’ve got the factory Y pipe from the Gen1 EJ22 on my L, so there’s no issue with the factory bash plate.
UELs in the style of the turbo exhaust manifold (with the pipe crossing under the front of the engine but usually reversed to the turbo setup) can be made to have little to no change to the bash plate. The trick is to keep it up high and possible make a heat shield for the cam covers.
To make a decent note you’ll need mufflers that don’t muffle the sound so well - straight through mufflers seem to do this well. A decent exhaust shop should sort this out no issue.
This is my setup from 9 years ago. It’s still running now without any issues even though I’ve scraped it, rocked on it and flooded it with water...
you can hear the exhaust in this video once it changes to the outside camera:
That said, the aftermarket UEL sound is sweet. We did some DIY ones for my sister’s Gen3 RX. The other day as it pulled into our driveway I was waiting to check out the WRX that I thought was about to drive past! It’s got a cat back “sports exhaust” from the previous owner, the UELs make it Subaru spec now. No video of that I’m sorry.
Pic of the build process:
and the final product - I forgot to get a pic of it fitted:
with this setup we can return it to stock if need be as we’ve not modified any points where joints occur in the exhaust, really though, why would you go back to stock??
Cheers
Bennie