"There is a difference between the best 155lber in the world and the most proven 155 lber in the world.
If you are ranking the most proven 155lber. You, can argue sherk not there (if you ignore his career at 170). However, if you are saying who is the best 155 lber, I think you need to consider sherk top 5."
One win over an unranked opponent (Florian) does not get you automatically ranked at LW.
But at the same time, I would favor Sherk over pretty much every other LW out there, except maybe BJ.
But rankings are not supposed to be predictive nor based on potential over actual accomplishment.
When Randy dropped down to 205, he was already a former 2-time UFC Champion at HW, but he wasn't immediately ranked at 205 based on his announcement alone.
He was ranked only after actually dispatching Chuck, a top 3 LHW, and ranked top 2 or co-#1 (with Wanderlei) after beating another top 3 LHW in Tito.
While Franca may not currently be ranked, he is definitely a formidable opponent who used to be a top contender at LW. An impressive win over Franca should probably enter Sherk into the top ten.
And if he actually does as well as his potential predicts, imo he should become a top 3, maybe even the #1 LW in the world.
But that ranking has to happen actually after that reality proves itself, not before. Rankings should follow accomplishment, not the other way around.