Why not make the 45mm ts-e faster? If you can make a 90mm ts-e f/2.8 you can make a 45mm ts-e f/2 or 1.8
The reason for the smaller aperture is the need for a design that creates the image circle necessary to adequately cover the sensor when shifted. The 90 has a natively larger image circle and thus can be a bit faster.
While it may possibly made faster there is scant practical purpose as the quality when shifted at large apertures would be poor and the cost would be very much higher.
TS lenses are almost always used on a tripod and one can still reduce the DOF be tilting the lens to get a narrow DOF at relatively small apertures. The idea that one might use it as an all-purpose lens is a bit of a stretch as MF in dim light is a pain. Macro ability might be useful though as long as infinity focus quality is not impacted.