It may be too early to decide on the exhausts- things will become clearer as the colour-scheme for the frame & the rest of the bike is done. However, do you need to have much silencing... I'm thinking that one look that may work would be to have 4 pipes (maybe with small baffles inside) coming straight back from the cyl head separately & parallel, then each left and right pair having a single bend outwards and upwards, under the seat part of the seat unit, then swiftly exiting as a pair of pipes either side, like how the pipes exit on a top fuel dragster. It's not easy to describe this kinda thing, is it? Can somebody mock up some in photoshop?!