I don't think he was up there with Roger and Whitebeard. In militarity might, he was probably the strongest, but he has no individual feats to stay with the top guns. Sengoku even puts him in the same shelf as Captain John, which is somewhat disappointing IMO.
Ace novel put him directly above Prime Linlin and equal to WB and Roger.
IMO, it was due to his military power if anything, he probably had the biggest fleet of the New World at the time. Besides, the novels aren't really that much reliable, since Oda didn't make them.
Chapter #0 and the panel above (chapter #957) are the only canon info we have about Shiki's power.
Strong enough that Roger needed a cop-out to escape him.
Strong enough that Prime Garp & Shiki were needed to defeat Shiki 2v1.
Shiki is HIM. He'll return soon
Roger was facing his entire fleet in a sea battle during a storm. The situation would be perilous for almost anyone (well, at least for DFless people... Whitebeard, BM and Kaido wouldn't have the same difficulty, they had good abilities for naval warfare). Plus, we can only speculate the result of a battle that never happened.
Garp and Sengoku 2v1'ing him means nothing too. I mean, Akainu had to play dirty against an already ill Whitebeard to finally face him, but it doesn't mean he couldn't fight him right from the start... he just used the best strategy to deal with him.