Unless one loads to a significantly higher pressure than the basic 30-06 case is loaded, none of the improved versions (to include the Gibbs) are really going to produce much of an advantage over the basic case. Using the ROT that the potential velocity at a given pressure level is going to increase by right about 25% of the increase in volume, even the Gibbs is only going to pick up about 60 FPS over the basic 30-06 case with (for example) a 200 grain bullet.
To get more you're just going to have to load to more than the 60K PSI the basic -06 case was held to (as you certainly can, and safely too). Which begs the question - why not just load the basic 30-06 to something more than 60K PSI. How can it be unsafe when it's OK for so many other cartridges with the same case head/bolt thrust?
I don't do it, but I wouldn't condemn it if there was a reliable source of pressure tested 30-06 "+P" data. I don't think one can tell 65K PSI from 70K PSI using traditional observable "pressure signs." How nice it would be to have some actual tested 65K PSI data for the 30-06.
Just my two bits,
Rex