I recall him implying that he was at a captain's level and then later speaking to Urahara. Urahara stated something like "I'm glad you're back at full strength, I'd hate it if you blamed me if you were weaker" and then Isshin just says that he wouldn't blame Urahara if he lost strength, he'd just accept it. So I think he did have his full strength that night.
It's safe to say that his power level is that of a high tier captain. When Ichigo finds out that Shinji is a vizard, during the convo they both remarked that the reiatsu they were feeling was extremely high. And for someone as strong as Shinji to say that says a lot.
Shinji also didn't recognize the spiritual pressure. So Isshin may be before Shinji's time. Ichigo having the same last name would be recognizable in SS and it wasn't sooo with that info he could have been around before Yamamoto but then again that just doesn't seem right. Isshin hasn't aged that much, he'd have gray hair and be old looking. Idk, KT really just likes leaving people on cliff hangers or mentioning things and then never explaining them.
Random thought but how the hell does everyone know Urahara anyways? He escaped punishment and fled to the real world yet everyone knows his location. You'd think SS would have tried to capture him during all this time. Granted his punishment was that he'd be stripped of his power and sent to earth anyways, still you'd think before he was such a help as he is now, that they'd track him down.
Something no one has thought to mention is that Isshin may not have gone by the same name he is going by now....might be why no one recognize it.
Also Urahara may not have truly come on to SS radar until recently which is why SS never bothered him