I'm about 70kg and I often go with the shorter mast with some healthy extension (18-40cms). While it does loosen up the top a bit, I find that with today's sails the top section of the sail provides more vent than drive. So on the scheme of things I'm of the opinion that a certain amount of liveliness up top doesn't adversely hurt performance. However, I have other sails where I'm using very little if any extension, and the performance is still great with the proper flex. So, if you have the 400 and the 430, it might be interesting to experiment a bit to see how you like the resulting performance.
More recently I broke a 400 bottom section, but I have a some tops left from earlier masts. Given the fact the 430 bottom section was only about 12-13cms longer than the 400 bottom, I did some experiments to see how things worked out. I put a 370 top on the 430 bottom. The overall length was in the proper range, but the result was a bit bizarre. Way too much shape in the bottom of the sail and the top was uselessly loose. Not a viable combination.
Yet, for a couple years I had great luck using the 400 base and the 370 top together, but the 400 and 370 bottoms were the exact same length with the 400 being slightly stiffer. Although this mix and match thing is kind of outside your questions, some different combinations are possible, as long as the differences aren't too great. So one could find some success matching the 430 bottom with the 400 top, or maybe the reverse using the 400 bottom and the 430 top. Of course, the masts need to be compatible a the ferrule connection.