Sorry to bump an oldish thread, but I had this exact issue. Vaccum was sitting ~9-8 at idle. Would only boost to 4PSI in 1,2,3. 4&5 would cap to 5PSI. Wastegate duty was completely capped on the AP at 90.2. AFR’s looked good, MAF Voltage looked good.
I’ve been trying to track it down for the past week.
I ended up replacing the IC and BPV with better condition stockers. checked and adjusted all the inlet, intake/couplers, IC, And TB hoses and clamps while I was there. Checked all vacuum lines and clamps, (ordered a silicone set because they’re all crusty on a 15 yr old car.) verified the restrictor pill size.
It ended up being the BCS. Verified with a buddies off of an 05 FXT. Now hitting 9-11PSI
Ordered for ~$110USD
Posting to hopefully save heartache for another owner with an old foz that might have been through a block or two