the economy of actions comes into play at this point. As does damage output.
at some point it's not increasing the difficulty in the same fashion that additional monsters would because solo+elite templated monster can only do one thing a round, and only does so much damage.
If you took a dragon, and added the elite template to it. I am fairly confident it would lose pretty hard to 5 equal level characters, as they would use dailies to get buffs running, and the dragons damage output would still be unlikely to outrace the parties damage/healing capabilities.
The templates add a certain amount to the monsters, I don't think they are designed to have more then 1 template applied to them, as that would increase their defenses to an unreasonably high level (as per just using a higher level monster).
If you wanted a very challenging encounter adding elite to a solo monster would probably be a reasonable thing to do. I would estimate it would be closer to raising the encounter level then raising the creature count.
If you look at what the elite template adds to the solo template. it's basically just HP, an action point, and some defense buffs. You want to give more uses of encounter powers, but how would you do that? I would probably take one of the solo monsters encounter powers and give it a recharge of 5-6 or 6. or add an extra chance to recharge one of the recharge abilities. (such as a dragons breath weapon that recharges on 5-6 make it 4-5-6).