I don't think this is going to work. IIRC, if you have a transform matrix that has scaling and/or rotation info as well as translation, the scale and rotation are applied first. So the translation components can't be combined independently, unless the scale and rotation of both SRTs are identical. Matrix multiplication isn't commutative, so you can't really expect to get an accurate result if you change the order inl which you combine the component transforms.
Yeah, you're right, I found out that if I apply the rotation of the first SRT to the translation of the second SRT before adding the 2 vectors, I do get the same vector as the matrix multiplications, give or take 0.000001 due to floating point error (scale is 1,1,1 so its mute at the moment, but I am sure I would have to take it into account as well).
My "rotate_vector_by_quaternion" packs quite a lot of operations, so now I am just pondering whether its worth it or not. In the end what I may be saving is a temp array of matrices since what OpenGL (and I suspect Direct3D) expects is matrices as well anyway.
Thanks for your help!