Nah, Straus' sub defense was too garbage for me to think he wouldn't have found a way to choke in the biggest fights. Skill wise he was better than both Pat and Patricio in his prime, but he had Sonnen syndrome in terms of snatching defeat from the jaws of victory.
McGeary had nice power and could be tricky on the mat if you let him play guard, but if he jumped into the UFC when he was peaking he'd have been lucky to hit the top 10 imo, same deal with Newton who had to rally in certain fights he was a favorite in and overall had a weird style that might net him some success for a bit defying the expectations before getting figured out and going on a skid, which is kinda what happened to him in Bellator.
Volkov we already know what he did, Minakov and Konrad are interesting since they were undefeated at their best, though Minakov faced stiffer competition and also had a pretty damn competitive fight with Kongo which makes me think he'd have topped out as a top 5 depending on matchups but never held the belt, HW contenders at the time were nasty. He'd beat someone like Browne though, easy. Cole is hard to say since he didn't love MMA and was pretty limited. Unless he developed some finishing instincts he wouldn't have been well liked by the brass either, I think he'd have been a perennial top 10 guy but faltered against the more lethal foes who wouldn't just let him ride them.
Of the old MW champs only Lombard and Shlemenko are worth looking at imo. Shlemenko I think would have ran into massive issues if matchup with wrestle/grapplers, especially back then, but against strikers he could've put up some nice fights. Don't think he'd have ever contended, but he'd have been around the top 15 and earned his fair share of FN main events imo. Again, MW was a shark tank during his peak. Carvalho and Halsey both have too big of holes to have ever made noise imo, though Rafael was more durable than Brandon so he could've at least stuck around on prelims or something like that, I think Brandon would've washed out in quick order.
WW is the most interesting one for me tbh, we had actual UFC contenders make the jump to Bellator while they were either at their peak (Larkin) or near it, plus a vet like Daley. Lima and Koreshkov do share similar deficiencies in the wrestling department that could've cost them, moreso for Douglas, but I think they would've stayed ranked for a long time if they had crossed over. Maybe not held the belt, but they would've given a pretty good fight to any of the title holders/contenders at the time post-GSP.
LW we know what Eddie did.... and Will Brooks

would've been fun to see Chandler as a younger guy in there though.
FW Pitbull and Curran could've tussled with the best, but I do think a decade ago we weren't seeing the best Pitbull so he would've taken some Ls as he moved up the ranks. Over time he really improved his striking game defensively (Straus and Weichel both picked him apart at times, though he was always able to score comebacks if needed) which I think really made him into the dominant force of FW everyone knew he was capable of being.
BW Dudu is the only interesting champ imo, and he lost his edge after the Warren loss. Tyson loss was also rough, but shit happens. If he was willing to be dynamic and aggressive then he could've had some nice moments, but if it was the Dudu who was just coasting, I don't think he would've gone very far. Overall though, lack of wrestling would've killed his title chances. I guess Caldwell counts as an old champ too, but I really don't know what to make of him. Especially not after his cardio went to shit.