Are you sure the Holosun (without the pic-rail adapter) weighs 2.65 oz? I suspect the weight of the 507C minus rail attachment is on par with the SRO @ 1.6-ish oz.
The Shadow 2 is 46 oz and change, so it’s already over the 45.0 oz weight limit for CO. Factor in 1.6 oz for the optic plus another 1 oz-ish for the basepads, you need to remove almost 4 oz from the slide to make weight.
A number of companies can do this work: CZC, Primary Machine, Fire 4 Effect, CGW... just depends on what you’re looking for. CZC uses a plate system. F4E has their own method of mounting without plate, as does PM and CGW now. I went with F4E, they milled my slide for the SRO and took off enough weight from the slide so the entire gun + optic + mag w/ extension is 44.1 oz (I replaced the stock AL grips w/ Lok grips).