There is nothing to suggest to us readers that he is as strong as stated dammit!
Listen, the problem with Hashirama is that we have not really seen all he has to offer. Most of what we know about him is through proxy (Tenzou, Madara and Zetsu). However, I think you underrate him because you believe that mokuton is the extent of his powers. The databook says that the reason shodai was the strongest man of his time was (1) mokuton and (2) bijuu manipulating powers. Once you take both into account, you quickly realize that shodai was a beast
Shodai Displayed Jutsu
1.
Mokuton -- is the most versatile jutsu. Enough to make Tenzou a jounin
2.
Kokuangyou no jutsu -- is an amazing genjutsu. Enough to make sandaime hokage a sitting duck
3.
Moku Bunshin no jutsu -- These bunshin do not dissipate when hit, they can merge with the ground and other trees for surprise attacks. They can be converted into chakra receivers and placed on the enemy for tracking
With simply his displayed powers, Hashirama can defeat most named jounins. His genjutsu is pretty scary and his mokuton was so strong, compared to Tenzou's, that he could restrain the freaking kyuubi itself.
Shodai Implied Jutsu
1. Tajuu Kage Bunshin -- The first databook says that Hashirama restricted this jutsu to (1) kage class ninjas and (2) ninjas with lots of chakra. Since it is clear that he was a chakra beast and he was the only hokage at the time and he sealed this jutsu in his kinjutsu scroll, it goes without saying that he had this jutsu
2. Regeneration -- We have seen all his clones use some form of regeneration. Tenzou regenerated his wound against Sasuke. Madara regenerated his arm against Minato and Zetsu regenerated his neck after Raikage "killed" him. Plus, the clone that Madara cultivated also regenerates limbs
White Zetsu Power
The manga makes it clear that white Zetsu is an alter ego of the shodai hokage and that the jutsu he uses are mokuton. So, it stands to reason that shodai hokage could use some form of his jutsu. However, the difference between Zetsu and shodai is that shodai was A LOT MORE powerful than Zetsu. Again, this implies that shodai could steal chakra from his adversaries, perform some kind of traveling jutsu comparable to kagerou (Tenzou something close to kagerou by the way)
Bijuu Power
Madara said that shodai controlled several bijuu and we know from Danzou that he had perfect control over those bijuu. Now, the databook clearly says that shodai was without equal because of his mokuton and bijuu manipulation powers. While we don't know exactly how he manipulated bijuu, we do know that controlling ONE bijuu is enough to make one kage level (Yagura, Kirabi, Naruto, Gaara, etc.). However, shodai hokage did not control one bijuu but several bijuu. With this power, he was certainly a beast. Whether he used the bijuu intenally (like a jinchiruki) or externally (like Madara with the kyuubi), he was a beast regardless.
Put that all together and you have a shinobi who can use one of the strongest genjutsu in the manga, can regenerate from wounds, can use tajuu moku bunshin and these bunshin do not dissapear when you hit them, can steal your chakra and on top of that had an army of bijuu under his control. That is superior to 99% of ninjas we have seen so far