I think the weight saving will be marginal as you will have to increase the tube diameter to match the strength. If not already done then I would look at data to find out what diameter and wall thickness of tube matches or exceeds that of the 1.6mm 25box.
Apart from that, and the complexity of ensuring the dimensions are correct the only other major issue is that round tube is more difficult to fix things to.
There is scope for development of the design, and in a way that has already hapend with the MX5 chassis. If using round tube there is also a lot of scope for using 1 piece tube bend sections for various parts of the chassis reducing the number of welds required.