Very interesting. I have a similar setup on my skyline. Full 3inch from turbo back exhaust, SAFC2, and bleeder set to 10psi, Stock turbo, and stock intercooler Boost starts dropping off up 5000 rpm, by 7000 rpm boost has dropped to 7.5 psi. But power still keeps climbing.
I also want to get a EBC to help hold boost but didn't even think about intercooler efficiency. If you put 1 boost guage before and 1 after the intercooler you could measure the presure drop accross the intercooler at certain boost levels. Eg. set boost to 12.5 psi (off guage After Cooler) by 7000 boost should be bakc to about 10psi. Watch the gauge before the intercooler to see what boost it is running if it is over 13.5 psi at any stage it would probably be unsafe to hold 10psi to redline with the stock cooler.