Hi Anthony. I have used a 7D with the 70-300mm L lens for the past 2 years, shoot RAW and use Lightroom 4.4. Zac is correct that focal length wise, your 55-250 mm will be fine.
Advantages of the 7D include the ability to adjust the back focus of your lens, which is critical using longer tele lenses. Mine needed some tweaking, & I have advised other photographers to check their setup, as, to date, they all were improved from the original setting. The focusing system when tracking fast moving aircraft is very good. In my opinion, you are unlikely to 'get to know' the 7D well enough with a weekend hire, & it's menu system will have similarities, but also fundamental differences from your 400D.
The 70-300mm lens is brilliant, as are most if not all of the L lenses. It's IS system is more modern than the one employed by the 100-400mm lens - there are plenty of good reviews on line about these & other manufacturers lens' suitable for aviation photography. Let us know how you get on.
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