The Keeppowes will trip at 8.3A, using them in the SC31 Pro at turbo looks like a bad idea. Use either a NCR18650GA for max capacity or something like a Samsung 30Q for more lumens.
The C8G may still work with the INR21700-50E, but it comes with a 4000mAh battery, it’s a sign that they believe it needs a higher drain battery, from the review it’s unclear if the step down is only themal or also from voltage. The C8F certainly will benefit from a higher drain battery. If you can wait for overseas shipping, buy from Vapcell a 5000mAh 50G (Gold) or a 5000mAh 50T (Red). Otherwise buy a Samsung 4000mAh 40T from Nkon or wait until the Molicel INR21700-M50A is in stock.