If you go ahead and ask most PH, they will tell you that the extra speed that the RUM don't offer you any real knock-down power. And that just the standard magnums can give you 90% of the balistics that the RUM give. However you need to keep in mind that extra fast bullets, tend to cause "some" bullet failures and thats something that all hunters hate to have...
check this out:
here is the taylor KO value for 300 win mag using 180 grain
.3 * 180 * 2960 / 7000 = 23.8
here is the taylor KO value for 300 RUM using 180 grain
.3 * 180 * 3250 / 7000 = 25
As you can see, the difference in knock down power of the faster bullet is almost too small to even be noticed.
If I was you, I'd get a long range round that some some REAL knock-down power, something like a 338 win mag or even a 375 h&h (with lighter bullets for the long distances)