Traded in my 2014 M6 392 SRT8 for a Hellcat m6 (previously had a 2011 6.4l and a 2010 6.1l) and the difference is night and day. loved the SRT8s (hence buying 3 in a row) but the Hellcat well worth the premium imho
The 392 Transmission is not rated to handle the 650 lb ft of torque that the stock Hellcat puts out. There are aftermarket transmission gear sets out there most notably Tranzilla which can withstand the torque of the stock Hellcat engine.
I had a 392 SRT...when I changed to the Hellcat, I was surprised the gear ratios in the trans were different. At first, I thought the 392 had 'better' ratios. But now, I don't. The first gear in the HC is taller, and the rear gear is too, so it's a tougher car to take off in. And, the 6th gear is much 'less tall'...so you pull more revs on the highway. But, the overall effect is the ratios in all 6 gears are 'closer' which makes for a better driving car. In the 392, 6th gear was really too tall so it was more like a 5 speed. And I drove it using 1-3-5 almost all the time so it was really like a 3 speed.