welcome to the forums!!!!
nah, most MPS's can get better than 10L/100km's on the highway quite easily, city driving is a killer tho... ugh, i got 12L/100 on my last fill up, damn sydney traffic... but got high 9's on recent cruise, which involved a bit of enthusiastic driving an hour or so on the highway home (~300km's all up) thats an MPS3, figure it'll be similar for a MPS6, am sure one of the owners of a 6 can answer better for you!
and as rogwick said, your fuel econ shouldnt be affected, whether or not you give it a bootfull it more post-mods is another thing entirely
