Why would you want to mess with the best shocks around. The Mugen shocks that came with my MBX5T are the best shocks I have ever seen. So Smooth, well made and they already have a large resevoir built into them. Honestly there is no need to add even that extra amount of weight to the Mugen. Besides, they wouldn't make that much of a difference to the shocks performance.
Way back when I had a T-MAXX, I had the Integy shocks with the resevoir on mine because I thought they would make it handle better. All they ended up adding was weight and some bling. Other than that they did very little for the handling.
I would much rather spend the $25 on another bottle of Nitro, than the shock resevoirs.