GT-40 compression

I bought my 99 air nautique last year and forgot to get a compression test just for
Baseline numbers. When I got it dewinterize this week the dealship did a compression test and was surprised by the numbers for a boat this old. 156 was the lowest and 165 the highest. I asked what was normal and they said 125. Is that right?I know the PO said they had done an engine rebuild but had no paperwork so I didn’t know if one had been done or not but the dealer said it’s definately not a standard GT-40 head and it looked to them from the numbers they were getting that it’s definatly modified and might even be a 302 head giving a smaller space thus a higher compression ratio? My concern is should I only put premium gas to avoid predetenation? They also said by how fast it revs it seems like I might have a modified cam as well. I bought new AIV to replace the old one as the engine sometimes gets into a loping idle that ends in either the engine dying or I give it enough throttle to get the engine above idle and then it smooths out and runs great. The dealship seems to think the ecm just can’t keep up with the modifications to the engines at idle. But I think if the ecm can make the engine run great at every speed but idle I’m hoping the AIV can fix that. I’ll let you know after I replace it and test it out on the lake again.