Replacing front satellites with 'better' speakers

Currently in the throes of a full system overhaul at the moment although only got as far as replacing my aging Marantz amp with a Sony 5200ES (fantastic amp ).  Once I've replaced my TV and stand I'll finally have room to replace my front satellite KEF2005.2 speakers with new speakers (was thinking floorstanders).  My lounge is 9' x 12' but mostly open plan to the dining room and the speakers will back onto the archway to the dining room partly.  I've heard that this kind of upgrade can lead to an impression of the centre speaker stuggling to provide the oomph for dialogue however.

 So I'm wondering, is this kind of upgrade generally considered worth it, and are there any pitfalls I should be aware of?  My budget could stretch to £800 or so, although that would have to include the new centre should that be necessary.  Saw the new Kef iQ5SE in this months mag and was pretty interested as a friend has the non SE ones and I like the sound of them, but am fairly easy with any recommendations, especially for centres as I really have no idea.