My outboard man reckons that is about right for 150hp engine.
Daniel, whilst I am no expert, I hae always been told 3/4" for every 100 hp. I agree with JW it does seem alot.
Out of interest I have just moved my 150 v6 Mariner (Carb version) back to the centre line of my Ribtec. No obvious differences and no vices from mounting the engine on the centre line. (The orginal offset was about 2" but the size of the first engine unknown.) If there are now no handling vices then suggest you leave it alone rather than drill 4 extra holes int he transom.
The Ospreys have a great hull shape and whilst not being as fast as a dart like Scorpion I know which one I would prefer to be sat on in a storm.
By the way the viper hull can be traced back to Fletcher, the makers of all those speed boat things.