1) Fossil fuels costs will eventually start rising and never come down again. How much investment and attention will synthetic production receive when oil is $100/barrel? Or at $200/barrel, or $300/barrel? More investment could lead to significant process improvements.
2) Solar energy has improved tremendously and continues to improve more. But the great weakness of solar is that it's only effective at daytime. Anything produced in excess of daytime demand is basically waste energy. Since it's waste energy, the inefficiency of the oil synthesis process doesn't matter. The only competitor here is other energy storage methods such as batteries. The key question here will be, is it better to store this excess waste energy in batteries or use the synthetic process to make oil? From a pure efficiency standpoint, battery is better. But when considering the existing oil infrastructure, the answer is not as clear.