Tony Halme(Ludvig Borga), Bam Bam Bigelow, and Mark Jindrak were all "beasts" that tried to step in and they all lost. I believe all three were better athletes than Sid. Just being 300lbs doesn't mean you can compete in MMA, it's the ones with athletic pedigrees that can make the jump. JBL, Ron Simmons, Vader (doubt he could hit 265), Droz, Brian Pillman, and Ernie Ladd would have been interesting "Giants" to compete in MMA.
Oh and Lawrence Taylor that guy could have been a HW champion, big as Sapp, fast as Lesnar.