It clearly is the typicall shounen emotional-power-of-friendship power up. is a bit difficult to put it into numbers. Kyoukai and Bajio also have some kind of alpha symbol which is supposed to be an even bigger power up than Shins.
Shins power up has to be at least 3 points maybe even 5. When he fights Rinko he has 88 STR and Rinko has 93. So 5 points would be obvious. But this would make the other alphas extremely high. If Bajio had 88 STR and manages to fight 99 STR Rankai with his power up it has to be definitely more than 5 points. I think it is likely for him to be around 95 STR with his power up. Kyoukai with a power up of 5 points would allready hit 100 STR.
So my guess is Shin (90 STR + 3), Kyoukai (95 STR + 5) and Bajio (90 STR + 5) :P
I'm pretty sure I read somewhere (probably in the manga, or simply something my mind made up) that the symbol gives Shin +5 in STR. And I think the reason Kyoukai and Bajio have it is because of their respective "trait", with Kyoukai being a Shiyuu and having that Chi breath ability, and with Bajio it's simply that animalistic side he has, which he suppresses normally. At least that's what I would attribute their alpha symbol to.