Flashlube does help with VSR but does not cure it in all cases.
The liquid injection does have some advantages and it should help with cooling the cylinders and valves but I don't think this will help much with engines that suffer from VSR. The early liquid injection systems had lots of problems and i suspect that the latest systems are also likely to suffer with fuel pump failure. I'm going to keep staying with vapour systems for quite some time yet but am watching the progress with the Liquid systems.
There are a number of factors that can cause VSR. Have a read of this link and it does cover most of the issues and is not a million miles of teh mark. I don't normaly agree with this guy but what he done here s not far off just with a couple of things missed off
http://www.ibadesign.info/LPG/VSR.pdf
What he says about the valve acting as a hammer is a chunk of where the problem lies but he has missed off a factor that is the make or break.
Lean burning is a well know problem amongest installers in the know. UKLPG have a list of engines that suffer with VSR on LPG. The problem with the list is that most of the info comes from two system suppliers, one has very poor fuel control and in the early days major injector failure and the other does have slight issues that with some engines will cause VSR due to leaning. A lot the problems will also come down poor installation. A good example is the 4.7 V8 jeep is on the list. There are 1000's of these converted with the common good systems and they are perfectly fine without any valve care.
I do not claim to fully understand VSR but I have a good idea and the reasons do vary from one engine to another.
The answer to the volvo 4.4 v8 is I do not think they will be a problem but the one I did I fitted with a valve saver system as I could not get enough data on the materials used in the engine and the details of the valve events.
I would keep to top quality parts and fit a valve saver system as insurance.
Damage caused by lean burning will not be resolved by any valve care products