I don't think there is anything you can do about this. The only possibility is to make sure the hinges are lubed properly but don't do anything to the struts.
Looking forward to the summer... I think you'll find the struts work more freely in warmer weather.
I don't find the boot particularly heavy but me mum always complains when she tries to shut it! I keep telling her it is still lighter than the Mondeo I had before the 2008 which she also always complained about!
I had weak struts on our Peugeot 307 and ordered new struts from these guys:
SGS Engineering of Derby.... 01332 576 850. ... WWW.SGS-ENGINEERING.COM
They were much stronger struts than the manufacturer's ones. In fact I had to stand back as they were so powerful!
I found them very helpful indeed and might be worth a phone call for advice and to see if they do them for the 2008
You shouldn't need to do it but if they say they are more powerful it would be worth £12.99 per strut.
I find that opening the hatch I have to lift it around 4-6 inches from the closed position then from there onwards it lifts itself.
I don't find the first 6 inches heavy personally but I do struggle to get the button right. There is also a short delay when pushing the button such that I am often trying to lift the hatch before it's open.
I would say that the delay between pushing the button and the boot releasing is about 0.5 seconds. It is very tempting to be trying to lift the boot before it's released and I do wonder if this is why it is often accused of being heavy. I would suggest that you push the button, wait for the boot to release then lift and see if it's still heavy.