Many good points made.

Actually the Spitfires got 5 bladed props of smaller diameter when the Griffon was fitted. The smaller diameter minimized the detrimental effects of increasing the power. One of many reasons why the Griffon Spitfire was a much nicer plane to fly than the Corsair for example, particularly near the stall.

The huge diameter of the Corsair's prop and it's consequent spiral airflow was the main reason for the Corsair having a wedge on the starboard wing to promote the stall.

Talking about blade efficiency is a whole can of worms unless you're going to go the whole hog, but on a model with a fixed pitch prop, or indeed a full size with fixed pitch prop then you can either absorb more power with more blade or more pitch.

Increasing pitch is fine for increased cruising speed but it will kill your acceleration and short field performance. Increasing blade size has already been covered, but to summarize, you need to account for:

Tip speed
Mechanical restrictions (eg ground clearance etc)

If you still need a good compromise of acceleration, top/cruising speed and not strimming the grass, then you need more blades.

Smaller diameters and more blades are generally accepted as being a quieter solution on full size aircraft, and result in a more user friendly aircraft from a flying point of view.