Some clarifications after reading posts re: Solar Drive/Hanamizuki on the tier list thread:

Wutan: Solar Drive damage still takes the enemy's defense into account in its formula, as seen here. I have this half-assed Solar Drive party which seems to deal more damage to, say, event stage bosses than to EX-tier bosses on steroids when firing Solar Drive at 300% gauge level. I affirm that enemy defense has something to do with this.

maotd: Indeed, Solar Drive Up isn't like most passives; it stacks in a multiplicative manner. That said, in a lineup of 5 bloomed rainbows, each with 70% Solar Drive Up, the raw multiplier is computed like this (with a little help from my friend Microsoft Excel):

x = [(1 + A)*(1 + B)*(1 + C)*(1 + D)*(1 + E)] – 1
A, B, C, D, and E correspond to the party members' Solar Drive boosts.

Given that all of the members in the above example have 70% Solar Drive Up:
x = [(1 + 0.70)*(1 + 0.70)*(1 + 0.70)*(1 + 0.70)*(1 + 0.70)] – 1
x = [(1 + 0.70)^5] – 1
x = [1.70^5] – 1
x = ~14.20 – 1
x = ~13.20

Multiply that by 100 to get 1320%, which is the maximum boost for Solar Drive (sans the boost from Light Gauge). I was able to confirm the said value upon watching Aqua Shadow gameplay videos from some Japanese whales where parties with 1320% Solar Drive damage took half of Mizukage Sakura and Ume's HP, just like in Wutan's screenshot.