On a stage one tuned non turbo 2.5L we get around 21miles to the UK gallon. (4.546L) Mod's include air intake improvement by way of second cold air feed and Nismo hi flow panel filter replacement, sports cat, mild exhaust mods, iridium plugs & Splitfire coilpacks.
The turbo of course won't deliver anything like that mpg, more like 14 or 15 mpg I think. Which is a good reason to go the tuned non turbo route if mpg is a concern. With the mods above you're not going to be far away on BHP from a standard or poorly tuned turbo (which many of them are) anyway, so down low to mid range there shouldn't be much difference, depends what you want it for I suppose. Ours is the wife's daily drive and she has an abject dilsike of turbo's, without that and if MPG is not a concern I see no reason to go for the non turbo.
Aitch