Info on liquid injection kits is a bit sketchy and where available may be a bit mis-informed / even hyped...
If you click to buy a liquid injection kit on a supplier site where listed, chances are the site will tell you there's no stock. Vialle was a manufacturer of these kits but we have been reliably informed they are no longer in business.
All model Cayennes involve compromise on LPG tank(s), because to keep the boot floor flat it is only possible to fit a small capacity tank in the wheel well / a cylinder tank can be fitted but will take up some luggage area / most of my customers have opted for 2 (one of each type) to be fitted, thus achieving decent LPG capacity with only a small cylinder tank in the luggage area. The liquid injection kits require an in tank LPG pump, not sure of the implications of fitting 2 such tanks, or pump flow rate (bhp rating), or of flow rate (bhp rating) of injectors.
I have repaired quite a few liquid injection systems. Yes they give a charge cooling effect but they don't offer as much flexibility in setup/configuration as a vapour injection system (ECUs) and for such reasons (and reasons mentioned above) I would't advise liquid injection, particularly on this model of vehicle.
Worth bearing in mind: With a vapour injection system you can fit individual LPG injectors for very short pipe runs to the inlet manifold. The liquid injection systems generally use injectors on rails, meaning longer pipe runs to the manifold runners (always best to keep pipe runs as short as possible, vapour or liquid injection), so the systems may not be exactly as most might imagine (guessing most would imagine they use something like petrol injectors mounted directly on the manifold with no pipe runs between injectors and manifold). The gas generally evaporates in the pipes between the rails and the ports, so the manifold still receives vapour but the vapour is colder than from the vapour injection systems.
There is space in the engine bay to plumb a vapour injection system in. At high engine loads LPG vapour injected will be at maybe the same temp as the air entering the engine (considering it's turbo'd and intercooled) and will constitute only around 1/15th of the total flow into the engine - like you said no charge cooling effect from injecting vapour (and you lose the petrol evaporation cooling effect) but won't be much of a rise in inlet temp and LPG has a higher octane (better anti-knock) than even the best super-unleaded. Fitting 2 tanks and running individual lines from them to the engine bay combined with adequate under bonnet components would allow the engine to run totally on gas even at full power, or compromise on components etc and have the system switch back to petrol when you put your foot down (which you probably won't need to do very often in a Cayenne turbo). A liquid injection system might even involve switching back to petrol when you put your foot down anyway.
Full time LPG installer
Servicing / Diagnostics / Repairs to all systems / DIY conversion kits supplied with thorough tech support
2 miles A1, 8 miles M62, http://www.Lpgc.co.uk