Yes you will get better MPG as you will be able to map the spark to suit your engine. It also works on the wasted spark principle sparking at the bottom of the stroke as well as the top. Lots of Maps around to suit your engine. I bet Rich's map will be a good starting point too if he's getting 55MPG. Another advantage of the Megasquirt/Megajolt and Probably other Systems like Emerald (I dont have much info on that one but Hans does) is that you can drive at Water Puddles and heavy without fear that you will have spark missfire as the systems are pretty well water resistant as long as you keep the ECU somewhere dry, (like inside the car, although in a Midas that is debateable).
I fitted an LPG kit in an E Class Mercedes about 4 years or so ago. I installed it over several days fitting a bit at a time. My wife used the car daily to get to work while I did it. I found that I didnt notice any reduction in power at all.I had to changethe plugs more often thats all. Main thing was to remember to let the car warm up before switching over to LPG as the solenoid can freeze without having the water system heating up the vapouriser..causing the air filer housing to 'pop'. The saving on fuel was great. I could buy LPG for 45p a litre at the local garage when at that time unleaded was about 95p a litre. My wife was commuting 65 miles each way (so 130 miles a day) and it was costing £50 a week in fuel. I consider that pretty good and the kit paid for itself in no time at all. Kit was about £450 back then. I just bought it from one of the LPG suppliers. WTV I think it was.
http://www.lpg-kits.com/. Theres others like the LandRover people at
http://www.tinleytech.co.uk although I dont know what they are like. The Merc used a toroidal tank (the more expensive one) which just fitted in the spare wheel well so you didnt notice it was there.
LPG burns very clean. Fascinatingly the oil that I took out at the oil change interval looked the same colour as it went in. There was no black death colour present. Some people reckon you can increase your mileage intervals for oil change because of this, although I didnt.
I have a spare LPG kit with sausage tank in the garage for an A series engine...using the SU intake. Just never got round to fitting it yet.
You can get pumps for pumping out from those 47KG propane bottles too (LPG is basically Propane/Butane). So you could even get your fuel delivered at your house if you have the storage. Cheap if you buy in bulk.
All the best,
David