I have seen this happen on a few occasions where the AFR readings on the rolling road bear little relationship to what happens on the open road in the real world.
On MAF sensor equipped cars I believe the problem is the way the air is fed to the induction system and MAF. The forced air feed which is very good at Dastek may not represent the actual air flow to the filter on the open road causing the sensor to miss read and give readings which bear little relationship to the actual situation on the main road.
It would be worthwhile, at the earliest opportunity, to attatch a wide band lambda up the tail pipe connected to an LM1 or similar to cross check readings during a full power run in 4th or 5th gear on a suitable road.
To answer the original question, the effect of running 12:1 or similar on full boost on the open road for 4 months...........well your engine won't last 4 months and it will be an expensive experience when a piston picks up in the bore or whatever. Getting a check as outlined above is only a 20 minute job for someone with the right equipment. If you were close to me I would happily do it for you.
Thanks Harvey
For the input and your offer to check the afr for me, but iam just outside glasgow so a bit to far...
Will get it checked again soon...