Just had a Milltek CBE fitted to my Mk6 Golf R and it sounds better than stock but i still want more!

Gonna get the downpipe but i have a few questions....

What brand should i consider?

Does it make much of a difference to sound over the CBE?

I've got a stage 1 APR tune, do i need to get stage 2 done before getting a downpipe fitted?

What about high flow cat? What difference to sound/performance will that make over installing just downpipe?

Cheers