My first reaction(and the first responses in this thread) was that one would simply need to check the strength of the elite to detemine whether or not stables was more beneficial than tg's.
This reasoning would lead us to assume that gnomes would be the optimal race to run stables.
I'll try to apply my formula to a gnome province close to oop.
Population
400 acres * (25*1.15) = 11500
Let's assume a draft of 65%
11500*0.65= 7475
Then say 42% of them are assigned to offensive roles.
7475*0.42= 3140
3140*5= 15,700 = raw offense
T= 400
S= 20
K= 0.0
L= 20/400 = 0.05
P= 1.0
y= 1.5 * 0.05 (1- (2 * 0.0 + 0.05)) = 0.0713
m= 1.09 * 1.076 (1.5 * 0.0 (1-0.0) + 1) = 1.1728
x= 15,700 * 1.1728 = 18,413
18,413 * 0.0713 = 1313
60 * 1.0 * 20 * 1.1728 = 1407
Stables are more effecient in this example, but only by a very close margin.
Any pop sci, raised draft or higher percentage of military units focused on offensive roles is going to push this back in favor of tg's.
..even for gnomes.