From your data, you have a very good combination now. What are you wanting to do/change? Here's what I found:
2017 Yamaha VF115 VMAX SHO Outboard Motor
Year 2017
Manufacturer Yamaha Marine
Model Name VF115
Trim Name LA
Generic Type (Primary) Four Stroke
Engine
Engine Type In-Line Four Cylinder
Engine Power 115 HP @ 5500 RPM
Alternator (Output): 35 Amp
Displacement 1832cc (111.8 ci)
Gear Ratio (Primary) 2.15:1
Lubrication Wet sump
Fuel Type Induction: EFI (DOHC)
Ignition System: TCI Micro Computer
Oil Type (Recommended): Yamalube® 4-M (See Owner's Manual)
Fuel Type (Recommended): Regular Unleaded (Minimum Pump Octane 87)
Capacities
Oil capacity 3.2 liters / 3.0 liters w/without filter
Range
Note Full Throttle RPM Range: 5300-6300 rpm
Dimensions
Length Shaft: 20"
Bore 3.19 in. (81 mm)
Stroke 3.5 in. (88.9 mm)
Weights
Dry Weight 377 lbs
Your WOT calculated slip is ~11%. That is pretty good for a pontoon and is great if you have a standard 3-blade ss prop...it's about as expected if you have a prop designed for pontoons (yes, Yamaha has those). Also your WOT RPM is in the desired range. Your current setup is good for a good top end and should accelerate well too.
Lifting the motor a notch or two "might" bump the top end (can't say for sure for your boat), but at some point that increases slippage too (maybe with even one notch). That would likely move WOT out of the desired range pushing you toward a 4-blade prop (if you don't already have a pontoon prop). Consider that you might be better served to buy motor wedges to get the bow down, particularly at less than WOT. Just 2 degrees of additional "tuck under" can make a big difference.
OK, all that said, here's what I learned with our combination: Tried all motor heights from full down to full up (drilled a couple more holes for that). Current setting is two holes up from motor sitting on the transom. All standard Yamaha 3-blade AL props except one (13 to 15 pitch) have 30+% slippage @ WOT at any motor height. However the good news is a switch to a Quicksilver Nemesis pontoon prop (4-blade AL twin of the Merc Spitfire). A 13p Yamaha 3-blade ran ~6300+ RPM. A 13P Nemesis runs a nominal 5500 RPM with better speed and, roughly, half as much calculated slip (yes, that's a drop of 7-800 RPM with the same pitch prop). BTW: I also now have about 2 degrees of additional down trim using a clearly red-neck solution to bow high low speed deck angle. For our pontoon, better deck angle reduces drag and gives just a little more speed at anything below ~4000 RPM.
If you still feel the need to tinker (you might surmise I would completely understand that), get some motor wedges to put in as you lift the motor a hole or two. Next, if you don't already have a pontoon prop, a Nemesis/Spitfire would be a fun and low cost experiment. I think you should consider Merc's general recommendation and stay with the same pitch IF you are currently operating in the desired range with a standard prop (read 3-blade). Your motor might be OK with a 15p pontoon Spitfire, but would likely be in the low end of the desired RPM range. Don't know about you, but I would be a little disappointed with that...
