Spur will be about -41 dBc.
In the absense of any other mismatches, nonlinearities or crosstalk you can calculate it by 20*LOG10[tan(IQphaseerror/2)]
Often such things are represented by "image" sideband contours levels on IQ amplitude and phase mismatch Cartesian axis.

No it is tan(phaseerror/2) not sin

It comes from the 10*LOG10{[1-cos(phaseerror)]/[1+cos(phaseerror)]} which is equal to 20*LOG10[tan(IQphaseerror/2)]

Look at Razavi RF microelectronics for derivation or

http://ieeexplore.ieee.org/iel5/4138799/4138800/04138864.pdf

A key thing is that in a SSB IQ modulator error vectors which cause the "image" sideband, i.e quadrature phase error and quadrature amplitude error, are orthogonal. Therefore you can null both phase and amplitude error independantly.