Since you are running the COS5 you should be able to utilise the alpha n (TPS VE Table) to aid in the tuning of this set up. This table was designed for the ITB or large cam engines with a weak map signal like you have.
If you log the GM.VETABLE_DMA pid once your main VE table is dialled in it will give you a starting point to tune this table.
Take a look here http://forum.efilive.com/showthread.php?t=2599 You will have to skip forwarda bit but it explains the process better than I could.