It`s hard to add anything big because you`ve already mentioned most of the key improvements.
Me knowing a lot more about BMWs than a sane person should ever know, I would suggest looking at all the usual tune up things like the condition of the air filter and spark plugs. After that I would advise testing your oxygen sensor, even if those things still work enought to pass MOT, they tend to worsen fuel economy.
Oxygen Sensor Information has a good procedure on doing it.
I thought of that because getting 40mpg on a 318is shouldn`t take much effort and with bumped up tyres and P&G you should get more.
Otherwise the first thing to do is get back the engine bellypan, doesn`t matter if you get the original one or make one yourself, it will not only improve aerodynamics and economy but also reduce road noise.
Other than that a grill block is the key to faster warm ups and more mpg and like you have found out yourself, the back bumpers do tend to parachute on these BMWs but some easy underbelly manufacturing can improve that. I doubt full underbody will be worthwhile if you do the front underpan and remove the back parachute.
As for electric fan conversion that is a great idea, just get a fan kit with electric thermostat that you stick into the radiator and throw out that old viscous coupling fan, those things are often the culprit for engine damage due to overheating and can be very parasitic on smaller engines. Going electric will not only get you better fuel economy, but also horsepower, easier revving and reliability.
You`ve also mentioned the diff. The 328i has a 2.93 diff and that would be the way to go, because the 318iS is very lowly geared for sporty driving thus economy suffers.
One thing that you may have not though of is replacing the engine DME chip. These improve low down torque and a 10% better fuel economy can be realistically expected. I`m not talking about the cheap resistors but about EPROM chips that can be found on eBay for a few tenners.
Please update on this topic with the results you will get, because I`m eager to find out how much can be squeezed out of a 318iS coupe. These cars are great if you keep rust away from them, lots of these engines go beyond 400k miles without major overhauls.