When you talk of XP per minute, I consider a whole 4 hours of play. If you do the missions faster on Tenacious, thats more time you spend traveling between contacts and missions. My guess would be that XP per minute would decline because of the time you spend running to and from missions and contacts. I do not have the time to test this thory, but maybe someone can deduce some conclusions based on this thought.
I personally think that if you do the missions on a tougher level, you could spend more time fighting and gaining better xp per minute. However, if you throw in the multiple mission bonuses that you could recieve in the time it takes you to do one at a harder level, would you net better xp? Please state what you think would happen.