The 8-twist should work for the DTAC 115s provided you have enough velocity. David Tubb has shot his 115s in both 8-twist and 1:7.5" twist barrels. He said the 8 will work, but the 1:7.5 is a safer best for all altitude/temp conditions.
Keep in mind that he's shooting the 6XC cartridge with velocities from 3000-3050. With a .243 you should be able to get the 115s past 3100,with most barrels). If you have a slowish barrel, and you're running the 115s in the 2950 range, the 8-twist may be marginal at low temps.
Bottom line: shoot them and see how they do. If you need to hot up the speed, one option is going moly, as that should allow you to run a bit more powder and drive the 115s to 3150.
Re the scope: Point Blank software calculates that, using a .585 BC for the 115s, at a velocity of 3050,1000' alt, 70deg F), you'll have 245" of drop from a 100-yard zero. That works out to 23.4 MOA. At 2950 fps, you'll have 266" of drop, or 25.4 MOA,1 MOA = 1.047").
Assuming you don't use too much elevation for your 100-yard zero,meaning the scope comes pretty well centered from the factory), you should be OK. Leupold says its 8-25 LT has 36 MOA of up above center.
A simple, inexpensive solution, BTW, is to use the Burris Signature Zee rings. These allow you to pre-load 20MOA into the scope rings. Are the Burris Rings good enough--well Jeff Cochran used them to win the 2005 F-Class nationals and the recent big regional match at WACO.