I just went through this with my 20' overnighter with a merc 200EFI and it can be a frustrating process to get it just right. One thing I learned is to first, satisfy yourself that your engine is running the way it should. My engine had badly fouled injectors so if I'd settled on a prop before fixing the injector problem, I'd have been way under propped and had to go through the whole process all over again.
One thing you don't mention is whether you're using aluminum or stainless props? I found that with my 200hp motor, a 19P Mirage plus was just the right prop and allows the motor to turn 5800rpm, near the top of the suggested range. So with your 50 less hp, you may be right that a 17P is a little too much. But last year, I was running an old 175hp carbed mercury with a 21P aluminum and it may have been slightly overpropped but still turned 5300rpm's, at the low end of the suggested range. I'm sure I could run that same 21P aluminum on my newer engine this year, but hole shot performance is much better with the Mirage plus. One thing nice about the Mirage plus is that it has 3 removable plugs in the sides of it to vent exhaust gasses just in front of the props blades, which allows it to quickly spin up to speed to get your motors RPM's to where the real power is, but as boat speed increases these vent holes are effectively closed off by water flowing by so you get the full effect of the blades pitch once you're up on plane and cruising. I think that's a pretty clever feature!
The best thing to do is to try different props and when you're talking $tainle$$, it's sort of tough to find a dealer who will loan them to you to try out. Then, if you do find a dealer who will do that, once you find the right prop, they're going to expect you to buy one from them at about $400-$500 each! But I found that one way to avoid that was to use eBay to procure props to try out, and then resell the ones that are the wrong pitch. I bought 3 stainless props, each in like new condition, for just under $200 per prop including shipping. Now, I've had the luxury to compare them all at my leisure and have determined which one is best for my boat/motor/load/use, I plan to resell at least one of the other 2 and maybe both of them so now I have confidence that I have just the right prop and will only have slightly more money in it than a new aluminum prop. That process will work for you too, but first make real sure that your motor is running just the way it should! Good luck!
