Dear Casper,
now only short to your point - as you can see Alexey is giving the example how to merge. I thing there is no conflict between ASAP and PRINCE2. The problem may be that this two are in some layers different and that differences may disturb.
The other questions is: are SAP implementations worth to be named "projects" or are they some kind of rutine replicable work? Are they still uniqual and so on what creates risk? If yes the use of PRINCE2 may strengthen the control. If not the use of PRINCE2 and in conequence the control is overdriven and the implementation has bigger cost.