It all depends on what you want to do with the boat. If you are only interested in running slow than a pair of 90s in a GB36 would be great. But the GB's semi-displacement hull can be run much faster than displacement speed. It takes power to do this, of course, and more fuel, but a lot of GB buyers, particularly of the newer boats, want to be able to get somewhere fairly quickly, cruise the area leisurely, and then get home quickly. This demand prompted American Marine/Grand Banks Llc to put larger and larger engines in their boats.
We cruise our GB at about 8 knots even though it has two FL120s. We have taken it to WOT one time to get data for the prop shop and IIRC the boat was doing some eleven or twelve knots (indicated). We stick to the 8-knot cruise speed not because we like running slow but because FL120s don't do well run hard.
If we have the boat re-engined as we have been contemplating for awhile now we will go with a pair of new 150 or 210 hp engines. If we do this, we will run the boat at ten knots or more if we can get it. This would be impossible with a pair of 90 hp engines.
But our priority--- given engines that can support it--- is speed. Other people's priority is economy regardless of speed. For this, a couple of 90 hp engines in a GB36 would a good setup.
If you are interested in putting the boat in charter, however, the low power may be a detriment. One of the GB52s in the large GB charter fleet in our marina was ordered with smaller-than-stock engines by its buyer. From what I have been told by the charter company, this boat has proved to be fairly unpopular with charter customers because it is so slow.