Dimitris, Welcome to ribnet!
Unfortunately I don't know your boat, or what the factory prop is so I can't give a good guess as to whether that prop is a good choice or not, but someone will likely be along later with a better idea. If you could borrow a portable tacho & find out the WOT RPM that will also help in the analysis. 1L/ mile is in the right area for most outboards.
One thing I can say for sure- more speed doesn't usually come hand in hand with less fuel consumption!
2x 25 Hp comparisons I can give are:
- Avon SR4 / Suz 25, 13" pitch std prop, hit 21 knots flat out on a good day, cruised happily at 18Knts.
- Waterlogged 11' Dell Quay dory / 1972 Johnson 25, factory prop (9" pitch) 16 knots flat out. and I'll work out the consumption one of these days!
Hope this helps.