There are really two ways that Meth injection can be used. It can be used as: A) purely as a tool to reduce the air charge temperature, or B) it can be used as an air charge reduce AND a fuel supplement. Both result in higher power numbers than without the Meth injection. If it is used as method B, then there can be some danger if something happens to the Methanol injection system (run out of meth, pump fails, etc).
If it were me preparing for upgrades and increases of power, I would go with the upgraded turbos now, run them within the limits of the OEM fuel system and then upgrade the OEM fuel system and take advantage of the larger capability turbos.
Since you are talking about 91 octane, that might change the picture a little bit as the Meth can be used in Method A, to allow more power to be made than what 91 alone can support. I hope to go down the E85 path, if that can ever be done with the DI fuel system only.