Would you mind taking the trouble to mention any 5x5 which provides 5 Forge Points and 5% attack and 5% defense and happiness and goods and coins and supplies and medals simultaneously?
Five SoKs take 5 squares less, I agree, but aside from coins and FPs, do they give an attack/defense boost or any of the above, for that matter? And look at it practically, on a patch of 5x5 land can you build 5 SoKs? No. (I am talking about just a 5x5 piece of land with no extra squares). You can build max 4 with enough squares left to logically be sufficient for another SoK or two, but practically not. You talk of buildings that are 3x3 and give a 10% att. boost. Fair enough, but aside from coins, again, anything else? I agree that an attack boost has far greater value than any of those, but then again, having some more of them won't hurt you, will it?
You can say that it has only a 5% att. boost, but don't forget that it even has a defense boost, and neither the speaker's corner nor the checkmate square has that. And 5% seems less, it can sometimes help in achieving victory for that hard battle.
For me, it is a very well balanced building if you look at it as a whole. Individually, yes, they might not be that strong but the point is to have them combined. But as they say, you cannot please everyone.