I have a .243 built on a Mauser 98. It is one of my sweetest guns...my pet rifle. BUT.....I consider it marginal when trying to drop very large deer....deer that dress out at 200 lbs and over. I have taken deer that big with my 243 but have been lucky that my shots were near perfect. As we all know this is not ALWAYS the case. We strive for perfect shots and sometimes get good shots, near perfect shots...occaisionaly though things don't always go well. I'm not suggesting that a bigger bullet can overcome bad shooting...but a bigger bullet can sometimes hold together better on bigger game. For most deer 30/06 is more than enough. For some big bruisers though it is not overkill.
A 30/06 would be a great rifle to have in your cabinet to compliment your .243