Like the title says, what down pipe manufacturer mates with an Sti cat back with no leak problem and uses the same gasket?

i think the helix bellmouth does

I was told Helix has a different gasket style that wont mate with an Sti

Cobb!

Invidia

I put an Invidia V2 catless dp on my 06 STi. It mated up fine with the stock mid-pipe.

Invidia FTW!!!!

Only dp with donut gasket will have perfect connection, cobb or MadDad. But you can also get shorty dp, such as storumug, oakos, bosal...