Nick you haven't told us what type of system you have. I'm running a direct injection (stag) system and am getting about the same as Bob when he's towing, but I'm not, so I still need to improve.
If you are due for new spark plugs go for a gas plug, they're a colder plug (I think).
I've been told that the TB patrols like to breath. I'm wanting to go with extractors and a 2 1/2" exhaust.
Other thing would a good tune if you can find someone how knows what they are doing..
What type of driving are you doing, motorway of city. That makes a big difference.
Good luck.