Street Unit is a legit vendor. They are affiliated with SURE Motorsports. Some parts that they make/sell as their own have some serious issues.
SURE Motorsports - Street Unit- StreetUnit.com - Design Flaws,Danger And Part Failures! - Mazdaspeed Forums
I've bought a Cobb downpipe from Street Unit before, didn't have any issues.
Also using the SURE 3" MAF intake and boost tubes and they are ok.
If you're just looking for regular Mazda/Mazdaspeed parts, check out EdgeAutosport.com instead.