There are a lot of claims about big velocity gains with Improved cartridges on the Internet. Most of these are probably loaded to dangerous pressure levels. Cases with little body taper tend to not display the typical high pressure symptoms (flattened primers, extractor marks, etc,) so people assume their loads are safe when they don't see these signs. There are basic formulas you can use to determine safe velocity levels when comparing standard and A.I. cartridges based upon added case capacity.

I only want to A.I. my 338-06 for the novelty. The velocity gain will be negligible over standard 338-06. Probably about 50 fps will be the most I can gain in velocity.