Why are you going for the torodial tank in a saloon ??
I have an 80 litre (64 ltr gas as you can only fill to 80%) in the boot.. anything smaller and you'll constantly be looking for LPG stations ... IMHO ..
I don't know what data you inputted to get the petrol/lpg "spend" figures .. but I reckon to save just under 10p /mile by running on LPG, so my conversion (£2173.68p .... two years ago) will take 24000 miles to "pay for itself"
You have to decide how long you will keep the car, how many miles you do in it, and how much you are willing to spend on the conversion, in order to decide on the viability.
If you DO decide to go for it do lots of research, ask garages how many omegas they have converted, and ask for contact details of conversions..... then ring and ask what the owners think.
There are many stories of conversions that have gone very badly wrong ... so be very, very careful.