OK - sounds like the hole has already been drilled in the plug. That gets to the WP screw/diaphragm.
What are you tapping in for the boost dump pressure? The plug?
Alan

Yip... just moving the plug in (the one with the hole for the WP). Was tapping it, but have since started to use a C Clamp, as its way more controllable!

Generally that won't control the boost dump. The base of the diaphragm is well beyond the plug. You can get a sense of that by how far in you have to insert the allen key to adjust the WP. Are you getting lower dump AFRs each time you push it in further? You can't go more than 3-4mm without it pinging free on the other side of the case.
I have modified mine externally - brazed a nut on the plug and have a screw I can thread in to the base of the diaphragm. The total movement of the diaphragm on boost dump is less than 0.5mm.
You may be chasing shadows with that one.
Regards
Alan

I've emailed you :)

K & Ns actually work better than a paper filter. After they've accumulated a little dust, which is what improves their efficiency over the paper filter. Takes a coupla 1000 miles or so.

If you keep cleaning it so that it looks bright pink all the time - yeh, not working as well as a paper filter, and maybe read the instructions that come with it.

In this specific case? ie a CIS 930 replacing the mailbox with the cone K&N on top of the air meter?

Because others have posted dyno data showing no change and I know I didn't have to make any adjustment at all when I did it.

frankencis ftw!

When doing dyno work developing my headers and mufflers part of that work was to preserve baseline by verifying any changes made such as air filters. The dyno we used seemed to have a margin of error range of about 5hp when corrected for conditions. I didn't see any significant difference in power levels when using the OE filter vs K&N vs none at all. This was on 300-400whp builds. So my take away was that any filter change difference was less than 5HP.

As filtration gets better with a K&N, dirty, flow goes down. Every thing is a trade off.

All the OP or any one needs to do to verify for themselves is to do no other change to the car and go back and forth with the stock air filter and K&N, does the F/A mixture go leaner? then try the K&N again, does the F/A ratio go richer.
I did on a few cars it changed just like the OP experience.
Filters of any kind reduce air flow and as they get dirty they reduce it more.

Thanks for the input everyone. Just to close this off, I have concluded that my old set up (modified mailbox) was actually more restrictive than a factory set up. With a 3" choke point, and a pretty dirty pod filter. Changing to the K&N directly onto the CIS inlet effectively put me back to where I should of been in the first place.

I've dialed a bit of fuel out of the on boost enrichment, and the car is flying!

The main reason I wanted to change to this set up, is for better access, and to give me a bit more room for a custom intercooler I'm about to start on.

Thanks again!