Hi mab, by the sounds your 4d34 is stock? If so there are quite substantial gains to be had, 1st thing is to open up the exhaust, 3" with a hotdog style muffler, including 3" brake if possible. Next is intake, in the stock intake pipe there's a bunch of noise baffle things, best thing is to replace the plastic vertical tube with a 4" pipe with ram head. Next is chip (ultimately more power needs more fuel), I had a DPchip which worked ok but did increase the idle speed which was annoying. Due to the semi electrical IP there doesn't seem to be an easy way to adjust idle manually. I believe chiptuning do one also, though I have no exp.
Don't forget that more fuel = more heat so boost needs to be upped to compensate. I just unplugged the wastegate line (intending to fit a boost controller) and it didn't do anything crazy so I just left it. Stock boost is 10, WG unplugged it ran 14 (maxed out) and with chip ran 16 easily.
Before chip + extra boost I got 5.6km/L, after it got 5.85. I can't give you any kind of factual evidence of how much it improved, but it was marked.
Just my 2c...