Re: how much more velocity?
The new batches starting maybe two years ago now of H-1000 are faster than the older batches I worked with developing loads for the 338-300 RUM back in the late 90's and the 338 RUM when it was introduced in 2001. Many guys work near max and the old 93-94 grain load with the 300 grain bullet could be very dangerous with the new H-1000 powder from what I hear. As I said I have not used the new powder but have read many times on here where it is faster by 3+ grains which is enough to be dangerous.
Other powders you need to watch out for are the old H-870 and accurate 870 which can vary by nearly 10% batch to batch and with temperature. Buy it by the keg so your load will be good for awhile and not have to change it with every pound of powder you open. H-1000 did well in a number of cartridges so I bought a few kegs and still using it. I think the new batches may be to fast for most purposes I use it for now.
Predictions are difficult, especially when they involve the future