I'm no speaker expert but the above reply makes sense, if you have to change the consraction of the speaker you will change the sound. It's not just the magnets and coils, ok you could boost up the power to 50W but the cones would probably need to be reinforced or they'd just deform under the load. If you change the power and the cones you've made a different speaker. Of course you can try to voice a speaker to sound like a higher powered version of a particular speaker, but I don't know how often this works?
As a side issue, 2x50w speakers wouldn't be enough to handle a 100W head. As rough guide you want your cab rated abut twice the power of the head (it's a very rough guide, it will change between every head / cab). The reasons for this have been gone into elsewhere, but in brief a 50w speaker will be on the red line of damage if run at 50w and will sound horrible as the cone will be deforming all over the place. Also a head can actually peak at higher than it's rating, if your speaker is already at it's physical limits then you can kiss it goodbye.
Also a head will be outputting most of its power by the time you are over half way on the gain and volume, so you can't assume that running a 100w head at half volume / gain will be outputting 50w, it will be more like 70 - 80w.
Edit: sorry got a bit off topic there.