At about 5ft 11in is the XL 58 Scott Addict R4 too big?

[HLaB]

I'm sceptical of fitting services that give you simple statements like "you should be riding a 58cm frame"

They're a bit more complex than that , they give several dimensions based on a number of factors like the type of bike, length/ angles etc. As you say though many manufactures dimensions are different and figures are often just a guide.