For me, the conditions are better. Any engine with the proper care is better in my book
15 HPs on a 100 HP engine is 15% more powerful the same 15 HPs on a 50 HP engine is 30% more, you will notice a 30% increase. The higher the output on a engine, lets say 200% 15HPs is about 7.5% increase.
I would say on these engines that runs below 100HP is a noticeable difference, not for much but you can feel it.
You could take your SOHC and add some off the shelves things here and there and gain the extra 15HPs.