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I think a good example of something like this happening was the "Dornian heresy" fanfic. While it's not canon at all, a independent and self sufficient force could develop their own ideology that is comparable with the tau and less dependent on the imperium. Trade and friendly relations could thrive there.

Obviously the armor and guns would improve for the space marines. The Tau would benefit by learning about genetic modification beyond the accelerated Darwinism they have now.
The space marines would know they were committing heresy and would be integrating with the Tau for protection of numbers from the inquisition and other space marine chapters. the Tau, as previously noted would be happy to get such experienced shock troopers.