I`ve recently bought a new Trek 1.5 which came with Bontrager 700x25 tyres. Would it make any difference (eg over 40km) if I changed them to 700x23`s?
negligible, the 25s will have a lower rolling resistance and will feel more comfortable, but there wont be a huge difference (or one that you would notice between 23/25.
However, there are a few more variables involved.
If you have got the money to spend tyres can be one of the best investments you can make, a decent set of tyres can save you loads of time, they will also be more puncture resistant.
So YES get a new pair of tyres, get the 23s if you want, i actually prefer them, pump them up to 120 psi, and away you go.
Spend approx 50-60 on a new set of tyres, it will be worth it trust me.
I used michelin pro light 2s, they are fantastic tyres and have one of the lowest rolling resistances you can get.
I have also never punctured in them and they are coming close to 500 miles.
michelin pro light/race (races have more puncture protection, but lights role better)
Schwelbe ultremo Rs
Veloflex records - the best if you can get your hands on a pair
Vittoria - chronos or corsas - both excellent tyres
Continentals are crap on rolling resistance but some people like them, personally with the exception of the gator skins I wouldn't touch them with a barge pole.