There are only two good uses for the scenario I'm envisioning (masses of HA, end game no later than knights) - either settle in another production city and figure that you will get the next XP reasonably easily or make an ubermash.
For a keshik/knight army the ubermash is it bit difficult though GK is one of the better leaders for it. First, you will need morale so it can keep up with the HAs. Next you need MIII/WIII - 7 promos. This requires 50 XP (or 65 using leadership), out of the box you should be able to get 7 xp from your military city and 20 xp from your gg. This gives you a grand total of 27 free xp and requires 23 fights at gimmee odds to promote further; using leadership requires 19 fights at gimmee odds. Depending upon map size this can be doable. However if you have your ubermash unit on a tile with another unit you can add another 10 XP each shot, burning 2 ggs will place you within 13 xp and 2 within 3.
If you feel like gambling, attacking at 89% odds (give or take depending upon first strike issues) you can very quickly get a leadership medic up to Morale/WIII/MIII with just about even odds of survival.
The settled gg is a surefire bet for more xp which is good. The ubermash with the bonus healing will eventually become more powerful as you have larger and larger stacks which are further and further from your main production cites.
I've played around with tactics/leadership mounted units, but I remain unimpressed with their yields. A mobility/shock axe is a complete waste; using GII archers and WII axes should be viable with agg/Arboria setup. Even if you can't a few shock keshiks should have reasonably decent odds against a spear on defense.
A big question is how much better a great general must be compared to a normal unit to be worthwhile.
In his arboria situation, a woodsman axe and some careful movement should keep up with keshiks. In the general situation, you might be moving your horse archer stack across flat land, at which point your archer/axe falls behind. You can slow down or risk letting spears suicide on your stack. Next turn you can clean them up outside of a city, but you will often be defending with your strongest horse archer.
How much do you lose per battle? I turn to a
combat calculator. Edit: the calculator doesn't subtract shock from defense properly. Used WB instead.
Spearman C1 vs Combat 2 Shock. 70.1%
Spearman 1 vs Combat 3 Shock. 66%
Spearman 2 vs combat 2 shock HA. 74.7%
Spearman 2 vs Combat 3 shock HA. 71.2%
Spearman 1 vs Combat Shock axe: 0.7%%
Spearman 1 vs Combat 3 axe: 0.8%%
Spearman 2 vs Combat 3 axe: 1.2%
Spearman 2 vs Combat 3 axe, 3.6 strength (average remaining strength after one battle): 20%
So each time a spear attacks, you lose about 0.65 to 0.75 horse archers.
Now they're not just ordinary 2 promotion horse archers, they are 3 to 4 promotion horse archers. I believe mithrandir is the one who said that a promotion is worth 15% more. So a 3 promotion horse archer is worth 11% more, a 4 promotion 23% more.
So each time a combat 1 spear attacks, your c3 morale axe earns about 0.8 worth of 5 exp horse archers. Every time a combat 2 spear attacks, you earn 0.82 to 0.87 horse archers.
So after say 5 defenses, you save about 4 horse archers. More if they attrition more, less if they don't. Is that worth it? Well if you're gunning from 11 exp to 17 exp, you're gaining a 11.5% bonus over a 5 exp horse archer (from the 15% figure) at the cost of 3 great generals, so each great general is about a 4% multiplier, or you get 4 horse archers worth after 100 horse archers.
A tactics great general will save 2.5 flanking 2 horse archers at terrible odds, possibly more with some promotions. Granted those are terrible since it's highly vulnerable to bad luck and they aren't replaceable...
Say a combat 6 great general is worth, let's see, say combat 6 is worth 2 promotions, 1+7*.15=2.05 compared to 1.3 for a 5 exp horse archer. That's 1.6 horse archers.
Note: on flat land, a combat 3 shock horse archer will defend a non-shock sword before a combat 1 shock axe, which is before a combat 3 axe. A combat 3 axe will defend before a combat 2 shock horse archer. Against a shock sword, the shock archer will defend first.