EL vs UEL sound completely different, while EL is designed for TS turbos you can use it but like Ruso said you WILL change your powerband significantly.
They make UEL headers for the BRZ/FRS/GT86 and they're the same block/heads, just N/A not turbo and funny thing is with UEL headers they make MORE power than stock due to UEL design increasing EGBP slightly and making more TQ down low/more flow up top.
Can UEL headers on a turbo car have issues, sure can...but the EJ257 STi still to this day uses UEL because it increases higher RPM flow where as the EJ207 lacks the TQ down low compared to the EJ257 therefor the EL/TS reduces that issue.
You need to decide what kind of driving the engine will be used for most before you decide how much money to drop on changing from EL to UEL, EL/TS for city is going to be much more responsive than UEL down low...while UEL is going to have more rumble and have higher HP number up top (which the FA20DIT doesn't even rotate past 7k).
Last but not least, the MAIN reason they went EL/TS on the wrx with the FA20DIT is it allows them to pass emissions easier here in the US. The EJ257 STi (USDM) still has UEL because Subaru has chosen it to be there one allowable "performance car" in their lineup, which is also why you see the STi having a catless up pipe and the wrx is still catted in its up pipe. Most manufacturers are allowed to have one car designed to be a performance machine and they allow less restrictions in terms of emissions testing.