Hi Everkane,
While I think that an attack value of 5 is not balanced in terms of firepower available at that age (meaning that there are currently no other units with that value; hardly even units with a value of 4), I think your resulting unit with 5.1.1 for 40 shields and no other specialities is not more unabalanced in terms of the punch you get for your shield investment than other more powerful unique units. I think that it is, in fact, more balanced in this way than some other units that are already part of the game. I would not consider it cheating if you used it, because, while it is true that you will not meet significant resistance when you finally do attack, you first have to build your unit, which is not cheap, and then you have to march it slowly to your target, and you have to protect it or possibly lose it to counterattack.
Other units are more unbalanced and already part of the game. The most serious offenders are the Gallic Swordsman, the Berserk and the Ancient Cavalry. Even some more balanced units are still more useful than a 5.1.1 unit for 40 shields, like the Mounted Warrior (3.1.2 for 30 shields) or the War Chariot (2.1.2 for 20 shields), because they have a significant punch for their respective costs and in addition to this are fast.
Faster movement is much more than just +1 to stats: It gives you the the possibility to flee and gives you thus some invisible HP, if you agree to look at it this way, but perhaps more importantly: Your strategical options are much better, for offense as well as for defense. You can rush to the defense of endangered cities, you can outmaneuver slower enemies and be the one to attack instead of having to absorb countertattacks, and finally, on the grand scale of things, your conquests, once started, will be much faster.
The non-unique fast units of the ancient age all lack punch. So you need more of them to defeat the same enemy. This is the (sensible) price for you better strategic options. Not so the unique units that are fast. The Gallic Swordsman has punch and speed combined and is thus much more powerful than just the sum of both attributes. 40 shields is quite a deal for this Uber-Unit. (Compare it to the Knight with only +1 on attack and defense, but 70 shields!) And the Ancient Cavalry is a Gallic Swordsman with an additional Hitpoint which clearly shows that the designers who included this unit into the game were not very good players or otherwise not interested in game balance.
The Berserk has distant similarities with your unit: It is not balanced in terms of firepower available at that age (Attack 6). The only other unit with that value in that age, the Cavalry, comes much later. In addition, the Berserk has the Amphibious attribute which makes it even more powerful. It is probably balanced in terms of punch you get for the shield investment, because it is indeed quite expensive (70 shields). However, I would have made it a 4.1.1 with Amphibious for 40 shields or 5.1.1 with Amphibious for 50 shields - very similar to the Longbowman it replaces.
Conclusion: In a way, you could, in terms of game-balance, already consider yourself cheating by playing an unmodified game using the Celts, who are agricultural, which is an almost game breakingly powerful trait, and have the Gallic Swordman as unique unit; or the Vikings with Berserks; or the Sumerians, who are agricultural AND scientific AND get Spearmen for the price of warriors. As it is, your unit is unbalanced only in terms of firepower available for that age, not in terms of punch you get for your shield investment. On another note, you could use a unique unit that is weaker, but much cheaper, i.e. a 4.1.1 unit for 25 shields (compare Immortal) or a 3.1.1 unit for 20 or 25 shields (compare Archer). These would be balanced in both categories, I think.
Happy Civving!
Lamabreeder