Yes I'm sure that coreAVC is used because before installation I couldn't play mkvfiles in MP.
MKV is a video/audio container file (just like an AVI file) and has nothing to do with h.264 content. An AVI file may provide some video encoded in h.264 and a MKV file a DivX stream, there is almost no rule about which contains which. Perhaps you already had a h.264 capable DirectShow filter on your system but you weren't aware of its presence before installing Haali's splitter.
Thanks for the tip regarding GraphEdit, that surely would explain my problem since I have an integrated crappy graphics card.
The magic behind CoreAVC is that it doesn't rely on any kind of hardware backend. You should get the exact same performance using either a GeForce 5200 or a 8800.