David: you are almost rigth¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡ all is a question of mix-match rigth combination

The problem is not the fan units, the problem is the following: DYNAMAX was one of the first true effective tractor DF unit and they work well due was designed around available engines at the moment and basically the OS. The OS runs happy and safe if you don't run above 21000 rpms, he can go more but you will be working in the red line and 22000 rpms is almost sure you will break rod at any moment. Instead, if you use a Rossi 90 with a Dynamax, you dont have to run more than half throttle or max 3/4, due HP/torque of bot engines are different.Rossi can go higher than OS without problems and also last more than double of OS engine but you need to know how to deal with a Rossi or you are dead bird, BVM engine is not easy but easier than Rossi and a bunch of power also.
Going with DF units, this is not a matter of blades but a matter of pitch. Example: Dynamax cant absorb Rossi 90 WOT for long periods, something will break if you don't put attention(above example) due the DYNAMAX have 11 blades and low pitch, has a lot of static trust wich make easier for landings and take-off, RAMTEC has 9 blades but higher trust,VIOFAN has I think 7 blades and high pitch.So here comparing three units you are going from the hig static trust (Dynamax) to the highest dynamic trust(VIOFAN) RAMTEC is in the middle and is one of the reason why you need a lot of distance for take of and landings with VIOFAN but also is the fastest DF unit. I explained this in an easyest way and without formulas and also with my basic English, but is similar principle when you change props at your airplane from two to three to four, you gains something but you loose also something. In my opinion OS will not overheat with VIOFAN for overev, but I think(you didn't give details) one main reason is the pipe, OS doest like small pipes(they will built more HP but heat increase easy) JMP has the right one, if you use some different pipe or combination you have to play and find the rigth combination(is your time and your money) but also what you said for heavy load on engine could be true but not for more blades but for the pitch.Engine is under heavy load ant low rpms no air flowing around for cooling and also you are not able to change timing and engine is not working at the desiderable powerband range plus a lot ot other involved variables like fuel(hig nitro with low rpms and high load means HEAT if you dont have the rigth glowplug.)

I personaly I`m building a TRIMAIR F-86 and have the RAMTEC unit recommended for the kit. It comes with their own inlet and outlet matching duct-sistem.I asked for some details and they said: It works perfect with OS 91 with the right pipe,if you install a Rossi and know how to deal with them it will fly much faster due it runs higger rpm, if you use Dynamax or Viofan, go to Dynamax due the special conditions required for Viofan.

My airplane is not ready yet but I build a test bench for my engines and combination due I have a lot of Rossi 90 I ordered Rossi mount and what the said was true, I tested Rossi with different pipes and pipes lengths and also with OS 91, with OS is works really smoot but doesnt develope as high rpms as Rossi and will built heta as rpms increase with small pipes.(small pipes compresed more the engines)

FLYTICOUN, remember if you want to go to RAMTEC you need a bigger intake area than a Dynamax, if you still wanting VIOFAN, find the rigth designed inlet and outlet ducts if you want to obtain true trust-rpm combination. Dynamax can be re-pitch from people offering the service(you can find here in the forums) or installing a RAMTEC rotor in a Dynamax(I don't know details but know people did)