The MT400 lever isn’t great – it’s designed for entry level riders that are two-finger braking. I think your SLX levers should improve the lever feel massively as they’re designed for 1-finger braking and have servo-wave, which the MT400 levers do not. This is essentially a cam that multiplies the power that the rider puts in and should definitely help with power.
They should be pretty easy to fit, likely no need to cut the hose, simply unthread the compression nut and thread into the new lever. If you’re careful you be able to do it and only need to perform a top bleed with the funnel.