Hi,
Not exactly the same but I went from a Bigma to a Sigma 300mm F2.8.
Have to say I was really pleased. The extra light the lense can take in makes a big difference and I've shot in situations that would be almost impossible for my Bigma - under trees on cloudy days etc.
Even for small birds I don't regret it - I use a 1.4 TC so still retain 420mm (+ sensor crop factor). As the bigma is sharpest at F8 and 400mm then I didn't feel like I'm losing reach.
As an example -
The drawback, as you point out, is the weight.
If the money isn't a problem and the weight, both carrying and transporting isn't a worry - the more options you have the better!
TobyH