Do these exist?
I really like the idea of the CO2 inflators but don't fancy accidentaly putting about 150PSI into a road tyre and putting 40PSI straight into an MTB tyre is just a waste.

Learn the Rule of Thumb.

Do you need a diagram to tell you which way 'Lefty Loosy' is as well?

Are you trying to tell me I should be able to feel when my road tyre hits 130PSI? I commend anyone who can squeeze a tyre at 120PSI and confidently say, 'nah not quite, you need another 10 in there'!

you'll only get about 90 psi in a road tyre even with a 16g cartridge.

Thought they were meant to give you about 130g? The 12g cartridges are specced to give 90psi.
Is this just not the case?

Pressure chart here:
http://www.genuineinnovations.com/bicycle/mountain/proflate-16-clone-2.html

No they'll never quite get to the quoted pressure.

IMO CO2 inflators are a 'get you out of trouble' fix on the road/trail, you stick a whole cartridge in, then sort it with a track pump when you get home.

You may as well empty a cartridge, as they 'bleed' air anyway, you won't be able to inflate 2 tyres with one 16g cartridge to a sensible pressure, and if you inflate a tyre and try to top it up a few days later the cartridge will have leaked the remainder anyway.

You'll never get 150psi out of one 16g cartridge, even on a 23C tyre. If you're worried though, get something like this: http://www.thebikechain.co.uk/store/product/25650/SKS-Airgun-CO2-Pump/

You can regulate the flow, so no need to pump it all in if you don't want to.

Why would you want 150psi in a road tyre! I've never found a 16g cartridge insufficient, they'll do about 90, which is more than enough to let you finish the ride.