Your maths is almost correct, 1200x1.15=1380, so 1400 is just over the 15%, though I couldn't say if they will let is threw because its so close and still an a series, best bet is to call RTA Technical Enquiries on 1300 137 302 and talk to them before going for the blue slip.

If you have to go to an engineer he will check that the car complies with the relevant ADR's and as long as its generally stock he should have no problem in signing it of.

I'm not sure what Tyrie has been told about the cross member, though cant understand why this would be any indication of it passing as its just a structural part and would just have to meet the structural requirements (that is just my assumptions and could be wrong).