I am probably not the most informed person to comment on this but...
There are several very good resons for using the bulkier powder. Powder position at the moment of ignition is important for shot to shot consistancy. This will result in lower extreme spreads in your velocity readings.
With some powders reduced loads are considered dangerous. I have been doing a lot of thinking about the post from IDShooter on the ignition problems with what apear to be starting loads with IMR 4350. (i am not familure with loading for the 300 Rem Ultra Mag). Ignition problems can result from powder position with slow burning powders.
Charles Petty had a very interesting article in Handloader a few years ago about the"solving" of the "SEE", Sudden Explosion Effect by PMC. PMC experianced a huge pressure spike due to a bullet stopping in the throat of a 6.5 X 55 cartridge.
I would be very worried if I had any cartridge with ignition problems.
Back to the subject, slower powders can also cause an increase in muzzle pressure, the pressure at the muzzle when the bullet exits, this can increase velocity while actualy decresing chamber pressure.
Interestingly, most powder charges are 90% consumed in the first two to three inches of bullet travel. Think about that. Peak pressure does not last long with any powder. Lloyd brownells series of articles in Handloader years ago were compiled into a book called Firearms Pressure Factors (I think) by Wolfe Publishing), You can probably get it on an interLibrary loan, good book.
Muzzle pressure is just as important a consideration in choosing the correct powder for your application as is breech pressure. This can be verified by looking at the load recommendations for contenders chambered for rifle calibers. The loads generating the highest velocities usualy require the traditional rifle powders rather than the fast pistol powders.
The reason, in my opinion, that many beginners have trouble with cast bullets is that they are using fast powders in big cartridge cases, and they experiance leading. A starting load for jacketed bullets would probably have given better results.
I'm sure there are people here with more experiance than I have on this very interesting subject. This is an important area of disscussion for the short barreled Guide Guns and Timber rifles.