Are there any kits out there that inject LPG in liquid form so that when it changes state to the gas, it removes the latent heat from the charge air (turbo application) and has the effect of cooling the intake and with high octane rating as a bonus? :) The injectors in such a system would be similar to typical petrol injectors (ie not a venturi carby system).

Also, do any of these kits allow you to remove the petrol fuel tank and petrol injectors?

I worked for a forklift OEM for 4+ yrs. They used GM and Mazda engines converted to LPG in 90% of the trucks sold in the USA. The only changes they made to the engines for LPG was installing valves made from better material (especially exhaust) and fit valve(Balance Valves) rotators to get acceptable life. Be aware that there is a small amount of tar in LPG and it can leave nasty deposits on the valve stems.

They did not use petrol to start/stop the engines nor have I ever heard of this practice. The idea behind dual fuel systems is you use the fuel that is most economical or available.

Using gasoline/petrol to start an engine brings several benefits.  First, it keeps the injectors and pump operating for a few seconds, flushing any debris and keeping the fuel fresh.

Second, it helps with cold starting.  Older mixer style systems may not start easily on cold mornings and using petrol to start made it easier.

With vapor injection, there has been a concern about getting a slug of liquid past the vaporizer on cold start and freezing the system.  Some use petrol start while others do not.

With liquid injection, these injectors take the place of the petrol injectors, thus, using petrol to start is difficult, since the petrol system is removed from the vehicle.  There are a few bi-fuel/liquid propane injection systems around, but not many, not that its impossible though.