If there's no visible leak and your going through a litre every 500 miles then its safe to assume that either the piston rings are worn or the turbo has seen better days, it may be a combination of wear on many parts and the engine could well be overdue for a rebuild. On a lighter note , all cossie engines use oil to a greater or lesser degree depending on age and driving style but not as much as you've quoted, dont worry though just keep topping her up. As for the cost of a stage 1 , well all you need is a chip and an adjustment on the standard actuator to run 15 psi and push 260/270 bhp. A Chip will not cost much more than £20 or £30 secondhand but a word of warning, if your motor is worn and using a lot of oil then its safe to assume that the oil pressure will not be the greatest and upping the power on a tired engine by another 50 or 60 bhp could prove terminal, not only to the car but also your wallet.....ah the joys of owning a cossie