On the other hand, people can be coached to significantly improve their IQ test scores. At least so says Rushton, of all people.

I'd point out that this should be extremely obvious a point, given how some subtests are Gc-loaded. You can 'improve' your IQ by studying some vocab, quite aside from the usual practice effects.

(And one of the standing questions about dual n-back is whether it doesn't (partially) amount to training for matrix-style Gf IQ tests.)