We have 2 engines like this with friends,one on a Solent Rib 750 the other on a batboat 26, both with hydrotech stage 1 kit, they have only 140 hours they are quiter than 250 opti, good torque, run smooth, yami's parts are not cheap, gearcase with regular shaft size instead of fat shaft on mercury XS or later optimax version, but the VMAX is pleasant to use, Both engines are very well maintened and look almost new after years.
Have heard they don't like hi rev, and hard offshore use . Finding Used parts for these engines are far more difficult to find than for opti, be carefull. The power head is pretty heavy too, and in hard use the saddles suffer a lot as they are not strong like racing XS series.
But puttting a VMAX 300 on a P21 is not reasonable in term of weight distribution, the COG of powerhead is going to be very high, and the transom will suffer a lot, you are going to have stability problems . If you plan to use the boat on mirror falt water all the time why not, but for me this good engine is not a ggod choice for a P21. JMHO...
The conversion af a 25'' opti to 20" is not a simple work to do, especially if the vertical shaft of your gearcase is in one part on the 25'', at this point you need to open the gear case and check gears backlashs, shimming, you need special merc tools, or some mecanical skills, the middle section, casing is very expensif too, bettter to find a broken powerhead on a 20'' an swap the powerhead to the 20'', making one engine with 2 in fact.