I've seen this said frequently. Better than what? IMO it is better to be on or about 13:1 A/F @ WOT, if possible. The 270's seem to like it a bit leaner than that for max power. In practice, on the street, if one is in the 12 - 14 range @ WOT consider it good.
When I say "slightly rich", I should say/mean on the rich side of the acceptable range of 12-14:1, ie. 12.5 instead of 13.5. From idle to when the mains start working,if you want power, IMHO,(from what I read and experience) especially for an air cooled motor and with carbs with NO accelerator pump you want it on the rich side! You can go slightly(I mean on the lean side of acceptable range)lean on the mains for better fuel economy (at that speed{vehicle speed}there should be enough air flow to keep an air cooled motor cool)
