Both R7 250 and 240 have the same architecture so it won't be matter if you are going to crossfire with 7700k.
So yeah 250 is better than 240 and 7700k igp with 250 have the same shader processing units. If you are willing to crossfire this APU and R7, there are performance gain, but not as good as doubled 250. Crossfiring igp and dedicated gpu is the cheapest way to boost performance but not as much. If you already have the card, then you can go for it and see what happens, but if you didn't have it, consider to buy higher performance gpu, such another R7 or maybe wait for a few days till RX 480 releases.