Fi's are pretty much the Forum boner right now. The SSDs are great for a daily triver setup, and several people on here have them and love them. Ngsm13 has owned both the Fi SSD, and the TC Sounds TC-1000. according to him, the SSD is superior in all ways.
As for the power handling, it all depends on how you set it up. If you get the copper coil, your SQ will be better, I wouldn't go over RMS spec for power. If you get the flatwind coil, you can increase power handling, but it won't have as good of SQ as the copper. In all honesty, i would go with the copper coil. You won't be able to tell much of a difference between 800 and 1000w. And even if you get a 1000w amp, after you set the gains right, it won't be putting out 1000w unless you crank it. A 1000w amp isn't putting out 1000w all the time.
As for single vs. dual sub setups. The 3dB gain thing is if you double the power, not the number of subs. Think of it this way: 2 10" woofers have around 157 sq. in. of cone area. One 15" woofer has around 176 sq. in. of cone area. 2 12s have about 226 sq. in. So 2 12s > 1 15 > 2 10s. As long as all other factors are the same.