Mercedes sprinter- low performance (please help a fellow diesel mechanic!)?

Is there a way I can pick up a bit more power/acceleration on my 311 cdi? its a 2001 and I have owned it for 4 weeks. In that time I have fully serviced it, replaced the front spring/ bottom ball joints/ front discs and pads. it had 2 blowing injectors so I removed them, cut the seats and fitted a full set of recon injectors, its fine in the 1st 3 gears revving up to 3750rpm, then once 4th is selected, its struggles a bit. It will pull 76mph but 5th really dont do much as it will eventually wind 71mph in 4th if I floor it.
Once I replaced the injectors it felt a bit flat and before I had done the work it would pull 80mph just now 76mph..I know things have to bed in but I was loosing a lot of compression on No 3 so expected a bit more grunt once done, any suggestions or can I do something with remapping or replacing the high pressure fuel pump. The van has done 199'000miles. I am not after top speed only over taking and pulling power. By the way, the glow plug light does not work at all, even on start up, I replaced the plugs at the same time as the injectors as a matter of good practice but the onces I took out was fine and it starts no problems even in cold mornings.

Im new to CDIs and Mercs so my knowlege its sketchy at best but been on the spanners fixing diesels for years but looking for advice from lads who have been working on common rails.