after reading countless post Im thinking of getting a helix downpipe for my wrx.

I have a few questions,

1. Does the helix downpipe fit perfectly to a tanabe hyper madellion cat back exhust?

2. does the helix downpipe replace the 2nd and last cat?

3. I know there will be a cel, and buying the dp at gruppe-s they give you a cel fix. How critical is having that fix? Do I have to wait till the cel comes on before applying the fix? or can I do the fix at the same time I install the downpipe.

4. lastly, I know to replace the stock dp we have to remove the heat sheild, my friend who replace his downpipe had to trim the heat shield and now there is only the heat shield for the turbo and none for the downpipe.. is it critcal to have heatshield on the downpipe and turbo or just the turbo? any aftermarket heatshield?

5. actually one more question. Even with the cel fix, is there any long term downside to fooling the o2sensor like that? or is it something very minimal and I should worry more about oil leakage than problems with life after cel fix.

thanks ^^
your help is most appreciateed *thumbs up