There are a few ways to look at it. One is that the powders and bullets of today in cartridges like the .270 Win, .280 Rem, and .308 Win. can pretty much match what 7mm Rem Mags did a few years ago.
A few examples:
This spring I tried a few loads in my .280 Rem. I was able to get 2,950fps with a 150gr bullet after a few different powders. One way to look at that is it shoots a bullet only 10grs lighter than the 7mm/160gr combo at about the same velocity. Pretty good!
I also tried Alliant's new powder, 2000-MR, in my .308 Win. Guys were raving about it on the internet in .308s, so I had to give it a try. I was able to (safely) get a 150gr Barnes TSX to 3,050fps. Again, one way to look at that is it is shooting a 150gr bullet only about 100fps slower than most 7mm Rem Mags.
The Berger/long range crowd likes to use the ultra-aerodynamic Bergers, etc in mild cartridges to get good results at long range. I can remember seeing a chart where the 7mm Rem Mag with a Berger actually surpasses the 30-378 at some point in energy, due to the Berger's high B.C. Now that comparison was done using standard flat base bullets in the 30-378, but it is still impressive. Heck, there is a video John Burns shot and posted on youtube of a girl knocking over an elk at 600 yards with a Berger in a .243 Win!
Of course, ALL guns get better with certain bullets. But if you're used to a "regular" 7mm bullet/load, you might be able to match that performance in a milder cartridge with newer powders and bullets.
I've heard of several guys getting over 3,150fps with a 120gr bullet in a 25-06 using Retumbo powder. That is a pretty good improvement over what was previously the top velocity for most powders in a .25-06 of 2,950fps. I bought some 120 Nosler Partitions to try in a .25-06 my grandfather gave me. I'm seriously thinking about working up a load and using it this fall. The S.D. and B.C. of those bullets are pretty good, and I think they would handle a buck out to around 400 yards easily. I might even try it on an elk too!
