I really don't trust on "higher batteries".
Battery is a thing that, if anything gets wrong, your phone could be damaged.
I agree with damaging your phone, but if you choose a good trustworthy manufacturer and stop hunting ebay for cheaper deals, then you have nothing to worry about. On the contrary well selected extended battery increases overall performance and with average use (mails, photos, twitter, blabla) your phone wont need any recharge for at least several days and in many cases its quite useful, depending on the lifestyle
amongst manufacturers with good reviews there is Mugen for example, those dudes even built a strong
fan community, users satisfied, no phones damaged, i even read somewhere they have the lowest returned marchandise rate amongst aftermarket batteries, whatever it means...
So dont worry about higher batteries, go for it guys, just buy from official sources for warranty, then its all dandy.