First of all 5900rpm is way too high for that engine. It should be revving to 5500 rpm max. This isn't a bad thing though, it just means that you'll gain more performance by fitting the correct prop.
The Trophy Sport is a 4 blade prop. 4 and 5 blade propellers versus 3 blade props usually:
- plane the boat faster.
- Keep the boat on plane at a lower speed.
- Improve mid-range speed at the same RPM
- Provide quicker acceleration than most 3 blade propellers
- Operate with less vibrations
- Have better holding capabilities in rough conditions
- Are less likely to ventilate in (sharp) turns
- Provide better low speed handling
- Are not quite as fast on the top end as a comparable 3 blade propeller
This should solve yor turning problem, now you have to drop the revs. Upgrading from a three blade to a four prop generally drops revs by 50 to 100. So this should get you down to 5800RPM. Also the change from Alu to Stainless should drop your revs even more because of the added weight (I am not sure on this one so maybe somebody else who has gone from alu to stainless can advise!!) so lets say you are now at 5700RPM
You said above that you do not want go any faster, but running a 13" 4 blader will in fact slow you down. Although theoretically any two 13" props should travel 13" in one revolution, a 4 blade prop is actually less efficient than a 3 blader and so will travel a shorter difference and therefore be slower. Changing the the prop pitch to 14" should enable you to keep your top speed and will also drop your revs by another 200 to about 5500RPM.
This is within the correct WOT range for your engine!! So if it were me I would be going for the 14" Trophy Sport (48-878616A40). Double check this with your Merc Dealer obviously!! In theory it works but I don't want you to go shelling out your hard earned on something that doesn't work.
Plenty of bargains to be had in the states!! Don't bring it back as hand luggage (I know you'll be tempted!) I would imagine a prop can be quite sharp when flung out of an overhead locker, OUCH!!
Hope this all made sense!! (or have I confused you even more!!)