Nahhhhhhh, 1/400 scale is HUGE ... compared to 1/700 scale, or those really insane guys who build ships in 1/1200 scale and even smaller.
Seriously, though, I think mixing scales at that degree is going to make the photoetch a dominant part of the model rather than a nice complement, because it is probably going to make peopel scratch their heads and try to figure out what's "wrong" about it. But then again, it's your model, so if you think it'll look good, go for it!