We had this issue on my neighbor's propane converted 8/1. Flow is determined by temperature drop, flow restriction, and the vertical drop or distance.
We had to go to a deeper Honda radiator to increase temperture drop, increase tubing to 1" to improve flow, raise the radiator as much as we could (lower edge of radiator 3 feet above the valve cover), and increase our wind turbine/convection tower to 16" diameter, about 7 foot above the radiator total. A fan is only used for backup and is rarely needed.
With the respectable temperature drop of a fan, and a 25-2??, I'd still stick to 1" pipe and the deeper honda radiator. I'd skip the thermostat except for switching the fan to improve flow. I'd try to keep the radiator at least 2.5 feet above the valve cover. If you're really into compact, the pump might make it easier and simpler, and you could use the cheapest honda radiator and smaller, cheaper fan(s). This is just my guess, and I would certainly defer to a 25/2 user with first hand experience for a setup similar to your planned one. I think on the 25 hp size I'd just go with a pump, myself.