If a Contractor wants to use a Subcontractor with a slightly different design but conforms with the Works Information and a cheaper option, with option C and with X15 do they have to declare the savings?
X15 is irrelevant in this particular situation. It depends if the design is your responsibility or it is the Employers. If it is Contractor design and they propose a value engineering idea, then this is an exception to 60.1(1) being a compensation event - meaning this value engineering idea will NOT change the total of the Prices (target) and both Parties will benefit from the saving. If the alternative subcontractor/design complies with Works Information there should be no reason to say no (and why would the Employer if they would benefit from the saving?)