Assuming the same length chain, let's think about the edge cases of front and rear chain ring/sprocket combinations:
Big front/Big rear: You hopefully don't ride in this combination
anyways but the RD cage would be pulled forward, but as it's already relatively inline with the chain there is no real difference..
Big front/Small rear: Short cage would normally hang downwards,
longer cage will take up more chain, it will therefore pull slightly
more forward. I wouldn't expect an issue.
Small front/Big rear: First gear, again, short cage that would normally hang downwards, longer cage will take up more chain, it will
therefore pull slightly more forward. I wouldn't expect an issue.
Small front/Small rear: Again, not a good combo to ride in, but in
case you do, having a longer cage will actually be helpful here as
the relatively shorter chain length will pull the cage forward.
All other cases would be between these extremes, that is, some form of 'it's going to be fine'.
Going the other way - using a short cage RD with a chain set for a long cage - may not work so well in case #4, but in that case a solution would be to remove links (much less potential for complication than when adding them ;)