[8X56MS]

I have loaded for a Marlin 336 in 30-30 Ackley for many years. I have found that its quite possible to push a 150 grain RN Hornady to well past 2600 fps. PO Ackley opined that the imporved cartridge case held tighter in the chamber when firing than the factory design, and therefore could be loaded to higher pressures. I have no extraction problems, or signs of excess pressure.
When I use Sierra Game King 150 grain Spitzers, its quite accurate at 200 yards. Of course, when using Spitzer bullets, the Marlin is treated as a two shot rifle. With no crimp, the Spitzer has a tendency to push back into the case. I admit to loading just a smidge long, to allow for that.
Overall, if I had to decide today, to rechamber the Marlin in Ackley Improved or not, I probably would not. While there is a real gain in 'oomph', there are other cartridges that still do a better job, and with less hassle. Working with Wildcats is not always about efficiency, or improvment, but playing with them is always a lot of fun, and a challange.