we bought a 5 furrow KV on no 28s last year, still got the old 4 furrow on no 8s for occasional use when biggest tractor is on the combi drill. 28s are definitely better for wide tyres, we did get away with 520s and no 8s though 16.9 tyres fitted better. No disadvantages I can see, if the 4 furrow ever does enough work to need new boards again, it will get 28s.