I don't agree with the 80-85% performance figure quoted above at all - the one I've seen elsewhere, and seemed right when I ran an LPG vehicle was 2% drop on performance. Ask yourself whether you'd really notice / be bothered by a 2% loss in power? 80% fuel consumption sounds about right though.
That issue of range is, or can be, the biggie. Gas tanks don't "fill to the brim" like a liquid fuel tank, so if you've been told it's got a 50 litre tank, assume you can get 40 litres of fuel into it.
You will need to know where your local LPG supplying garages are, and a bit more planning is required for out-of-area journeys as you'll want to know where you can fill up. You can buy a little road atlas that tells you where all the LPG stations are, and they send online updates through the year of changes. I only ever nearly got caught short on fuel once, but once was still worrying and if the effect is that you have to travel out of your way to get fuel, that can be PITA itself.
This is likely to be more of an issue on a smaller car where the tank mounting options are more limited, not so bad for big 4x4s where you can have big tanks. Also, assume that you will no longer have a spare wheel, which might be a nuisance if you need it.
I had LPG on a 4 litre Jeep, I was happy with it, apart from having to stop for fuel far too often, but I haven't missed the faff of finding stations since I switched to a Diesel car.
You shouldn't have any technical worries about a professionally fitted Prins multi-point, they're good (that's what I had!). The filling procedure is a bit more faff than petrol / diesel and there's a couple of different pump types out there but, to be honest, once you've done it a couple of times you'll be fine, it's not THAT hard...