They can be done and I have already done pretty much what you are trying to do using the mod switching feature and the unpacked files available on this forum.

I basically set timescale to 0.5, inceased the building build time by x4 and lowered the farming output of regions by a factor of 4 (based on the region chainges in Wonderlands SloMod). I did not alter the unit build times.

I am on my first play through test at the moment using Medium campaign difficulty and switching the FOW off to see how it affected game play.

I have assumed that things should be similar to the default balance and gameplay and so far it is. Though there are some differences. There is a little more avaiable money as there is so long to save up between build projects. But due to the slow population growth income in not rediculous. In fact it is difficult to get any population growth with very high taxes and no farms. There is a lot more time to build troops and moving them around and the AI seems to really take advantage of this. A nice effect for the French is the slowdown in build time seems to have prevented them building loads of Siege engines so I am (playing the English) facing french armies that are basically half cavalry and half spearmen...

I have played 50 turns (25 year) and no faction is wiped out yet though the HRE are suffering and Portugal is hammering in the nails for Spain's coffin. I have been forced into vassalage to the French by the Fanco-Danish-Portugese alliance. It was quite impressive, the French besieged all my continental holdings in one turn (except the one besiged by the Danes) and asked me to accept vassalage. The large rebels stacks in the UK and the weakened state of my military has basically forced me to conceed the point...

Anyway, like I said, this is the first play through...