The DCM isn't going to keep the 2-phase reaction at 40C if you add the acid extremely fast. It will help some, but since the DCM is not in solution with the reacting peracid and isosafrole, it can only transfer heat away from the outside of the polar liquid mass. The cooling efficiency will improve with the efficiency of the stirring because there is more surface area contact between the polar and nonpolar liquids. Since you say you were using an Ice bath, I imagine you weren't getting a good vortex going in the flask with your stirbar. What probably happened is the organic layer got incredibly hot and then finally hit a point where the DCM became increasingly soluble in it and was quickly boiled out the top. Since DCM takes very little energy to evaporate and the organics - water, formic acid, etc. - hold their heat relatively well, it would have been very difficult to contain the DCM even with a large condensor.
Don't take shortcuts unless you have a good idea about what you are doing. If you had pulled that fast addition bullshit with the classic performic using acetone, you could possibly be very badly burned and cut right now. Take this as a learning experience and try to get the basics down before you start dialing in your reactions.
All paths are the same: they lead nowhere
