I doubt if being a tier 2 company by itself will increase the SP...

I doubt if being a tier 2 company by itself will increase the SP to any great extent. What counts is the ROI and there is plenty of evidence to show how the SP of even tier 1 companies can get thrown around. I would think that behind the merger is a desire to grow the combined company even further with new projects and/or takeovers. For that more financing and capital is required and that is when the merged company becomes advantageous.