SLX cranks are great, but they are not carbon, nor hollow. Shimano make great aluminum cranks but don't make any carbon ones. XT SLX and above have hollow arms which are engineering marvels but crabon is lighter. When it comes to aluminium cranks, only Campagnolo can produce better aluminium than Shimano.

A bigger issue is chainline, as the SLX is MTB. A Shimano GRX crank would have a road chainline (which I would assume that the Sirrus has) and 40t chainrings.

Carbon cranks are fantastic for shedding that last bit of weight; but bang for buck, lighter wheels are a better waste use of money IMO.

Since Im editing this, I'll add that the engineering of Shimano bottom brackets has several advantages over other brands. Also, crabon cranks require the large torque wrenches (50ft-lb) for installation; something to consider for the DIY crowd. I have an SLX, a GRX, and a crabon crank (Easton) and I love em all; horses for courses.