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Westerner2,
In general the so called 'improved' chambers are really just a hobby. I have tried them in rimless, rimmed and belted rounds and now I am stuck with the odd chambers.
I even met Lysle Kilborn who invented the concept with the .22 K Hornet. He was an employee of Lymans.
For a 7mm-08 'Improved' chamber to be done right the headspace should be set to be a contact fit with the standard 7-08's neck shoulder junction. This is to make sure that the expansion web on the standard round does not get stretched making the side walls fail. This of course limits the chambers flexibility.
Not only all that but I doubt that you will get near 280 Rem ballistics with safe pressures. Keep in mind that there is little of any data for the subject round.
A common sense way around this is to keep or sell the 243 as it is and buy another rifle in 7mm.
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