Generally batteries have a life of 4-5 years. You get anomolies like 7-10 years, but those are the exceptions. So most likely its a factory battery and it would be a good idea to get a replacement. The sides bulging does not necessarily mean anything is failing but does hint to a 'well used' life. To figure out the exact date of the battery, you should look for a date code, which is normally found on the sides near the top burned into the plastic. It uses the first 12 letters of the alphabet for the month followed by the last number of the year(ie. July 2007 would be G7).
Interstate Batteries is a distributor of Interstate batteries only. They are a separate brand and design than such competitors as exide, duralast, walmart, etc. Yes they manufactured by Johnson Controls, but many of the companies you named and OEM replacement batteries are actually supplied by Interstate Batteries using different labels. Johnson Controls does not have a JC brand per se, to my knowledge.
Presently there are are the base models such as I-75s(65 month warranty), then Megatrons(75 month warranty) and the Megatron Plus(85 month warranty) when you are talking about lead acid batteries. You can get a megatron plus battery for about $75-$125 depending on the model.
If you are looking for high performance in regards to cranking power or cycle life, you can go for Optima batteries. The Optima battery uses a Spiral Cell technology that does not leak.(ie. placement options are endless). You said that you don't need the Optima but I figured I would throw out the information.
If you are running a relatively stock setup and want a good performer, I models or MT are a solid bet. If you want a top of the line model with the longer warranty, MTP is for you. If you are running a much bigger stereo( high output amps etc.), offroad lights, winches, or the like, yellowtop optimas are the ticket. High cycle life, high cranking amps, and a high reserve life. Period
I apologize for the long book of an answer, but seven years of the family battery business kinda leaves a mark. I hope it helps. Any other questions let me know.