Does the 300 hp IO550 conversion require an auxiliary tank on the Cessna 182?
This article gives a slightly dated (2002) overview of the engine upgrade options for a 182. There's no mention of auxiliary tanks being required, but it does note that for at least one option they're typically a good idea:


  The IO-550 engine is a guzzler and if you don't have long-range tanks,
  you'll need them. Indeed, you'll probably want to consider Flint tip
  tanks for your Skylane.


It's possible that you've seen conversions where, for whatever reason, a cabin tank was added instead of - or even in addition to - tip tanks.

If you're looking into an upgrade then if you aren't already a member you should consider joining the Cessna Pilots Association and search through their forums (members only) for whatever you can find. I found one thread on upgrading the C182 to an IO-550 but it didn't mention the fuel tanks at all, although I didn't search thoroughly.