Yeah you can bypass Red Herrings with Imp as Red Herrings triggers when you steal not when you access. Ash however initiates a trace that prevents anything other than him being accessed so you cannot use Imp to trash any other cards in the server.
Hope that helps
Right, and just to be explicit about Red Herrings, even if you use an Imp token to trash Red Herrings, you still need to pay the additional cost on that run, as Red Herrings is explicit that it applies for the duration of the run even if you trash it:
Red Herrings wrote:
Each time the Runner accesses an agenda from this server, he or she must pay 5 [Credits] as an additional cost in order to steal it. This applies even during the run on which the Runner trashes Red Herrings.
There is no concept of "bypassing" upgrades, and the effect of Red Herrings does not require the runner to access Red Herrings itself in order to be in effect.