G'day leeenfield:
apologies for not responding earlier, been away from my computer travelling.
Firstly, the neck thickening is not really excessive when sizing .308 down to .287, whereas sizing .375 down to .287 certainly is! As mentioned, there is a slight 'step' inside the neck where the shoulder started on the .300 donor case, but its only a thou or two and doesn't seem to be an issue.
One could always inside neck-ream anyway if the need arose, assuming one was suitably equipped to do so. I am up to 3rd firing on my Norma cases and no issues so far. The rifle will be warmed up again tomorrow in a Classic Light Nitro event, providing Cyclone Marcus drags the wet weather away to the south-west.
Regarding the effort required to size down the .300 Win Mag cases, I am pretty conservative and at no point did I consider the 7/8 die to be at risk of cracking. Two things I consider crucial to the process are: use of anhydrous lanolin as the case lube (commercial lubes don't cut the mustard); and pushing the case into the die a mm or 2 at a time, with ~180 degree turning of the case between compressions.
I did try neck-and-shoulder annealing the .300 cases first but found no discernible difference when using new Norma cases.
With the CH4D die, removal of the depriming rod allowed the neck to lengthen with no issues, trimming to length afterwards. Didn't worry about the loose necks for the fire-forming loads. With other dies, you may need to trim the .300 slightly beforehand.
Hope this info is helpful.