Stables add nw, and more importantly take time and acres to fill... acres and time that you might be using for guilds or banks during peace. The point is that thinking of Stables as the equivalent of TG is a flawed analogy... if you want to run stables instead of TG it works, but it is a really minor difference with a lot of tradeoffs. My personal stance would be to find the acres that let me run more than 20% TG+stables, or forget about heavy offense mods and focus more on econ and defense buildings. Just building average TG or stables puts Elf in a position where they're still inferior to pure attackers, but lacking other things to compensate for it.
Playing Elf/Sage as a heavy attacker with emphasis on offense power outside of war means that they can better translate that offensive power to Learn and Plunder gains, while being in a good warring position should the need arise. Elf's native economy is not going to be competitive enough to sit and pump for long.