You are indeed right, the subs will wire down to two ohms to the amp and the total combined RMS power of the subs is 900wrms, and the amp will put out a theoretical 870wrms.
Filtering out what other people have said in this thread, putting 870wrms into the subwoofers will be fine
Lets rule out some myths here;
Under powering a subwoofer will cause it to blow - No not at all, what will blow a subwoofer is DISTORTION. This is achieved by setting the gain too high on the amplifier and causing it to send out a 'clipped' signal. This is where they signal isn't a sine wave anymore and is more of a straight pattern with upto 90degree edges. Sending 100wrms into a speaker designed for say 2500wrms is fine as long as the signal isn't clipped and full of distortion.
You may hear people will blow their subwoofers up with an amp that was less powerful than the subwoofer, this is mainly due to one main reason, the higher the subwoofer ratings go, they generally tend to be less efficient (this isn't always the case but most mainstream woofers follow suit), so in order for the sub to play louder, people will generally push the gain pot further not realising they are sending through pure distortion.
If you want more info let me know and ill carry on.
Overpowering a subwoofer will blow it - same as above, it will only blow if its sent a distorted signal. Obviously, there are limits and considerations to take here, will a crap JBL subwoofer rated at 200wrms take more than 500wrms, its doubtful for extended periods, however, its not the voice coil that will let go generally, its usually the mechanical parts of the speaker - spider, cone, surround, dust cap etc. However, give something like an Ascendant Audio Havoc which is rated at 1500wrms, 3000wrms of a perfect sin wave and clip free, and it laughs its head off.
My old Ascendant Audio SMD's were rated at 3500wrms, I gave them 9700wrms each daily use and they got HAMMERED! Never had an issue.
Again, let me know if you want more detail.
Swapping the amp from x wrms to another amp that does 50wrms more, will I notice the difference - Highly unlikely. The difference in power only really makes a difference if you are going from say 1000wrms to 1500wrms. As a rule of thumb, an amp needs to produce approx 250-300wrms more power before you hear the difference and even then, the difference may not be as big as you think!
What makes the biggest difference when swapping out amplifiers, are the amplifiers themselves. Use something like a JBL/Boss/Pioneer etc and then swap out to something like a Mosconi, Gladen Audio, LW Audio etc, if the power ratings are the same or similar, the main reason you hear the difference is because the quality of the components used on the board are far superior and produce a 'better' sound. A bit like why you choose a R32 over a Ford Escort...its just better!
Personally, if it was my setup and you haven't bought the goods yet, I would pick slightly different gear but thats me!