You are correct - the whole "won't extract in the heat" issue is a leftover from the cordite days. That is not a problem with modern ammunition. There may be an issue with a particular rifle's extraction, but the issue won't be ammunition behaving differently in the heat.
Living in Texas, I find the persistence of the whole heat thing fairly amusing. Was reading some guy in the last month, talking about the dangers of using some English calibers early and late in the African season for dangerous game. Yep, you guessed it, the cases may swell and jam the rifle at a critical moment. Unless the "heat" of sub-Saharan Africa is of some yet unidentified Alien origin, I challenge anyone to find a warmer shooting experience than a range in south central Texas in the summer (or Arizona, New Mexico - hell - even Kansas).
With respect to purchasing a double rifle - never think of it as an investment. Purchase the best one you can afford. Treat it like it is the only such rifle you will ever own, and should you someday decide to sell or trade it, you will likely maximize your return. In the interim, you will be using a dependable rifle that you may decide is a lot better than you originally though.