I don't know the answer to this but it's a slow afternoon at work so I have nothing better to do than add my tuppence worth based on completely untested hypothesis. My contribution to this forum is pretty spurious most of the time!
On the colour point, I'm not sure how this works exactly but if I assume EBC is something that defines the contribution to colour for a specific weight of grain into a specific volume of water for a specific time at a specific temperature, then the mix might be out.
500g x 200 = 200
vs
250g x 400 = 200
250g x 100 = 25
Total 500g = 225
Is that even right / can you do that!? Who knows, but worth checking.
If it is, you might want:
166g x 400 = 132
334g x 100 = 67
Total 500g = 199
On the flavour point, I would think that a darker roast would naturally impart a different flavour. Think of heating sugar in a pan - you go from melted, to golden, to dark, all of which taste different despite being the same ingredient.
Not sure to what extent the extended roasting changes the percentage of fermentable sugars either??