Korg SP250 and Kawai ES3 on have 60 and 64 note polyphony, which is a far cry from Roland FP4's 128 note polyphony.
The Kawai MP5 has 192 note polyphony, but it looks like a dinasour.. When was this model released? Looks to be at least 5 years old.
Kawai ES6 is more comparable with 192 note polyphony, but it only has 32 voices.
For me, the Kawai keypad isn't as nice. I don't like Kawai acoustic pianos either, mind you so I may be biased.
So then the Roland's. I did not like the "bottoming out" feel of the FP7 as in my other thread, but I did like the FP4's action.
The major differences between the FP4 and the RD300GX are that the 300GX is a stage piano more suitable for gigging, whilst the FP4 is more of a home piano where you can record straight using it's recording functions.
The 300GX does not have speakers, the FP4's speakers aren't very powerful either.
It'd be wise to invest in monitors either way you go.
I was thinking Behringer B2031A Truth Active Monitors.
Seems like good value: http://www.zzounds.com/productreview--BEHB2031A