To let you know after running dbccfreeproccache to clear out stale plans Navision performance for the processes mentioned improved dramatically, and sp_execute makes up for a tiny percentage of queries according to Confio. I had heard parameter sniffing was a possible issue in Navision, and this is what let me to clear out the procedure cache.

Whether it is coincidence or not is not confirmed, last week was period end and very busy compared to this week, however from the stats from 5 weeks ago I can still see a marked improvement.