Based on those numbers alone, I doubt the RX-8 will be faster than the RB. Furthermore, the 106" wheelbase probably won't make it any nimbler than the RB (95.5"). Not to say the RX-8 will be a total dud (I think it should be able to hold its own compared to other cars in its class), but it's really not meant to be the RX-7 replacement.
From a practical standpoint, it probably makes more sense for you to get rid of your RB (costs more to maintain, uses more gas, costs more to insure) and keep the convertible. That way you can have a "practical" everyday car (the RX-8) and a fun occasional-use car (the convertible). On the other hand, if you kept the RB and relegated it to "non-daily driver" status, you'd probably put a lot less miles on it, thus cutting back on your operating expenses. Maybe you should keep the RB.
Sorry, I'm not helping here...
As for the weight, from what I've read in an interview with one of the Mazda production managers, he claimed that the target weight was 1260 kg (2778 lb) !!!!!! I would be thrilled if they managed to pull that off. It's doubtful that they can, but I think I'll be content if they manage to keep it in the 2900 lb range.