I'm getting conflicting advice from several (all trusted) sources. I'm trying to work out whether to go down the airbox route (quite fancy the noise reduction and some extra bhp if that's what it'll do) or just fit a trusty Pipercross. For several reasons the standard Blackbird airbox is impractical. What I'd like to know is... how critical are the lengths of the trumpets and why are they of different lengths? (the BB has approx 70, 60, 70, 60mm). Has anyone had any experience with the 'off-the-peg' ones on offer from companies like Revierie? Too many questions to continue! Any insights would be very gratefully received.

Yes the length of the induction tract is very important. The trumpets tend to be different lengths on a bike to improve the torque range. These can be obviously changed if your tuning the engine. If your leaving it as stock, I'd keep the stock trumpets.

Its can become massively complicated and while some general advice can be applied against trumpet lengths, you can loose as much as you gain if you make the worng choices.

I'm going for the standard trumpets (keep same torque curve) with air box (a little quieter), in-line air filter (free flowing) and cold air feed from the nose (denser air, more power).

A well designed airbox will give more BHP over the usual socks type arrangement. However a badly designed one will strangle the engine and create less power.
I used an airbox just because it was easiest but if you have the choice unless you're planning on being very lucky or very clever (to work out all the variables) it might be better just to stick with the usual solution...

I had a TTS air filter with stock Yamaha trumpets, then removed TTS and fitted the standard Yamaha airbox. Definetly more torque.

All im going to do now i fit a K&N panel filter and remove the Yamaha item.

You sure you've measured those lengths right? Mine's got the longer two in the middle. But then I run the stock airbox as I already had a hole in the bonnet when my last air filter got ruined so I just fitted the airbox back on again.

As others have said, IF you can design/get a suitable airbox then it will give you extra power over a plain air filter and probably reduce some noise. But how well you can do compared to Honda I don't know. The main thing with an airbox is using resonant tuning to fill it gaps in the torque curve. I don't know what the bird does without the stock airbox, but the factory curve doesn't really have much in the way of holes in it so I don't know ow much it'll be worth looking into.

Well spotted! Actually my measuring was OK, what was in error was the plank that decided to fit two left hand pairs to my throttle bodies - giving me the long, short, long, short effect. Now all sorted thanks to ebay.