You guys have one hell of a site here 1st and foremost. I have had an R33 GTS-t in my shop here in the states and I have been reading thru this site a bunch. We mostly do SR stuff at the shop so I thought I would put my two cents in on this post.
To get a curve that looks like yours you have something else going on other than just the ecu. We regularly run S14 SR's up to 12-14psi on all stock bits (maf, 370cc’s,ECU). Our A/F ratios stay in the mid 11's. What kind of spark plug gaps are you running?
I agree that changing out the ECU to something you can tune is huge benefit and throw a set of 550cc's in there and you will make some good power. On my personal S13 I made 330 whp and 280ft/lbs @ 16psi on a S14 SR turbo, 264 cams, AEM EMS, 850cc's and a Greddy plenum. These numbers are from a DynoJet, what can I say it’s the silly Yankee standard.
If I can help in any way let me know. I feel obligated after I have shamelessly pilfered a bunch of good info off this site.