A tough question. I have a history of using lots of sails with around 440-445 luff on 4m masts. I'm 69 kilos. I think it depends a lot on the sail of extending this much is a good option or not. Generally, I don't think a too loose top os stuff like that is the problem. Rather the overall softer 400 mast make the sail "fold" easier in its mid par as a result of the mast bending too much sideways. How early this happens depend on the sail and wether its a probelm depends on the rider. A heavier rider i likely to load the sail up more and therefor reach this folding point more often. However, even for a light sailor like me you can reach this point if you sail with a kind of locked in powered up style.
As for the general feel of the sail, everything will be a tad softer and more reactive with a softer (shorter) mast, but the power and profila can generelly be tuned just as well with a softer mast.
Also the exact mast you use matter. With rdms, some masts are allready a big soft and don't generally respond that well to lots of extension. I use the Hot sails Maui Hot Rod series of RDMs and they hold up for extension very well. The GulfTech needle 100 iUsed before worked well too.