I think it depends if you’re getting a big discount on a full group.
The current Deore brakes are honestly fine as is the front mech, I’d prefer the rest to be SLX myself, and tbh, I’d have the SLX brakes too as I’m a tart. M675 SLX has got to the point where XT is really more of an aspirational jump than noticable quantum leap – there are comments that Shimano have deliberately upspecced the SLX group as the rise in price of everything else used in building a bike meant they saw it being a far more likely thing for most people to encounter on bikes in the up to £2000ish range.
There’s enough bits of SLX that are worth having over Deore that a discounted full group probably negates picking and choosing. There’s only about £60 in it between M615 and M675 at Ribble for a full group (although they’re out of SLX) and about £75 between the same groups at CRC although they’re cheaper for both.