Only way this will work is if the drops per person are the same. So say a boss drops 2 pieces in 10 man, thats 1/5. The same boss will have to drop 5 pieces in 25 to keep the 1/5. So, doing a 10 man raid with 6 bosses would be 12 drops in 10 man and 30 in 25.
If you do two 10 man groups you're losing out on a piece of loot per boss. So over the course of 4 weeks with two raids that equal eleven bosses you'd get 88 single 10 man, 220 in 25 man and if you did two 10 mans it would only come to 176 which is a loss of 44 pieces of loots.
To me that seems like far from the death of 25 mans.