If the only criteria was extra power it might be cheapest $/kW to simply remove the LPG. Whether that's the way to go or not will depend upon what the current system is costing in terms of power, how much, if anything, you can sell the parts for and how far you intend to travel on the petrol alone (km's/yr X yrs that you will own the car).
Yes, vapour phase injection will not lose power in the same way some mixer style LPG systems can but they're also expensive. It would be hard to justify on an economic rationale the cost of dumping the current system, that already gets you the fuel cost saving of LPG, only to replace it with a different system.
How much is a cam and exhaust, using the current LPG system, vs a vapour phase system with stock cam and exhaust?
Have you actually looked to see how much restriction the mixer is causing. While you don't have a MAP sensor standard they are both inexpensive (used) and easy to set up. Measure the voltage out of that: ignition on, engine off, as a baseline of atmospheric pressure; WOT on LPG (and petrol if you like), mixer in place and WOT mixer removed (disable the LPG so you can't get LPG flow without the mixer in place and add back in the air duct in its place as that will only be a few $$ to do). That will tell you how much manifold pressure you are losing through flow restriction with the current mixer.
You could use MAF voltage instead to make a similar assessment but it won't give you a direct measure of the pressure loss, only its effect.