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You do both though. You go to a testing place with a set of tests in mind and you find out if they have all the helmets you want to test in advance and/or you borrow them. Then, pending results of the test, you buy your own 'fastest' lid.
Then maybe you get some more cash, you do the same thing for position and then you find bars that let you ride that position.
If you have loads to blow at once, you can do it all at once. But most people do it iteratively and that's why lids and stuff come up for sale often. Someone tested again, found a new position and a new lid that worked better.
Consultancy doesn't tell you that x is definitely faster than y for most things on you or your bike - there's too many interactions and where there's not, most people already know the answers.
Like when I went to see Xav and every one of my ideas was the fastest. I gained nothing, other than the knowledge that some of my 'slightly annoying' choices were faster.
On the flipside, when I rode the Nat 12 last, I did just buy the newest NoPinz skinsuit and assumed it was faster because they said so. I didn't have the time nor inclination to test it.
hippy-
The point you keep missing is everything prior to a tunnel or velodrome test. You or I thinking up a set of tests to do in a tunnel is an incredibly expensive way to piss money down the drain, you'd get through that £10k pretty quickly.
Maybe helmets is the only thing you want to test then fine, but spending time and money on someone who, based on your specific aims, budget, current equipment setup, performance data etc etc etc will work through whether testing helmets is even worth doing let alone a starting point.
Consultancy doesn't tell you that x is definitely faster than y for
most things on you or your bike - there's too many interactions and
where there's not, most people already know the answers.Of course it can't, that would be ridiculous. Unless you find someone who can 'see' aero...
onyerbike
One of the usual suspects, most of them do consultancy.
Yes they will probably want to try to sell you products/tunnel time but you might narrow down that actually you are only worth focusing on. It's like buying a load of stems and saddles, trying them out, then going to get a bike fit.
EDIT. Ah you added more to your post. Yes position and kit are interlinked but unelss you want to spend 30 hours in the tunnel, you should probably just accept getting close enough on both sides.