Love the franchise, still play it every week (highly recommend SF 30th anniversary collection). This slot is strange. The four rounds of free spins are the bosses from the game. Multiplier increases per fight. I've made it to the 3rd round 3 times now using a few different characters, mainly the 3rd and 4th highest paying.
I've always collected, rather than restart the fight
Switched to Blanka, higher on the win big scale than Guile. Got to sagat again
Finally switched to Chun Li, the highest on the win big scale I've triggered the free spins with so far
I've played for about 4 hours on this in total and I'm nearly £300 up and that's always playing 20p per spin. I'm determined to get to the final round and beat Bison, the multiplier potential if you did it with Ken (the highest paying character) is unreal!