Davis used to publish a spec for the solar accuracy with a cable extension, but it wasn’t one that made much sense and they wouldn’t/couldn’t clarify it. FWIW it was “±5% of full scale plus 45 W/m2 per 100’” I think it’s safe to say that extending the cable will degrade the accuracy, but in my experience up to 40’ isn’t a big deal. Beyond that I would be a bit nervous.
Despite the higher price than phone extension cable I would suggest using the Davis cable which reasonably low resistance, does stand up to UV quite well, and you can be sure it will not result in crossed connections.