The sdr-iq was very good for its day, a good dynamic range and no spurs, but its not a quiet receiver.
All the newer rigs seem great at weak signal if you operate them in weak signal mode.
The 7300 does seem to have a lot of gain in default mode.
I have had the Anan 100b and 10e, the Flex 6300 6400m, the 3000 and 5000 (lots of spurs),
a few icom 7300's the ftdx 3000, the airspy hf plus discovery, the sdr-iq, the mchf clones, the Hermes lite 2 and the Icom 7600, along with the Elecraft k1, k2, kx2 and KX3 radios.
The newer Anan and Flex radios seem to have the edge these days, I have not had a K3.
I think the antenna might be more critical for weak signal work if the rig is in the ballpark....
A loop antenna you can turn and point to the signal and null noise is a big help.