There are two assumptions here that I would point out are highly questionable.
First: is Tevinter really that efficient? That seems unclear to me. We don't know very much about the way the Empire works. Is it meaningfully more efficient than the other states of Thedas? Also, what does efficiency even mean?
Efficient as in, it has not fulfilled what some would believe about free mages, i.e. that such a society is not technically possible without it ending badly. This is also the same Imperium that's currently at war with the Qunari, It's fair to say that they are in reasonable control of their society. If they were not, do you not believe that the Qunari would have defeated them?
Second: if Tevinter is more efficient, is this due to the use of blood magic or magocratic government in general, or is it due to factors that are not necessarily related to that? Is Tevinter efficient in spite of magocratic government or due to it?
First, I never claimed that it was 'more efficient'. Their success is due in large part to their slaves and their blood magic. Blood magic which is said to be corrupt/weaken the veil/summon demons. Not sure about your last point. But what's sure is that mages run that society and it is not in ruins.
Also, not a questionable assumption but a salient point I'd like to draw out: is efficiency the only aspect of government with which we should be concerned? If that efficiency comes with a price - say, the diminution or destruction of human rights - then is it worth it to pay that price? If Tevinter's magocracy is "efficient" yet that efficiency relies on widespread slave labor, why single out Tevinter as worthy of emulation?
You don't have to have mass blood magic rituals and slaves to emulate Tevinter. You only need a standing society ruled by mages. Which Tevinter is. They use extremely dangerous magic, yet they still stand. They are doing something correctly when it comes to handling magic.
Interestingly, the White Chantry doesn't seem to have a problem with 'means to an end' efficiency and championing human rights. The only rights that they seem to care for. They mirror Tevinter without the blood magic. I simply think it's possible to do the same with bm free mages.