Yes I had the same thought: better lens but not such a ‘better’ sharpness due to the ‘softer’ e-shift 5 according to many many comparisons with the 4. And yes to leave something that I know in perfect conditions for something I have no knowledge about is highly risky. I am tempted by a 620 whose owner asks basically the same price I was asked for the 990. The 620 is a more ‘stable’ model but I feel unsure about the ‘upgrade’ or which would be a ‘better’ upgrade between a 620 and a 990 equally priced. Thank you very much for your kindnessI wouldn't at this stage in the game bother with the marginal up from 520 to 990. 990 Used the newer eshift algo which some say robs sharpness, so what you gain in lens you might not keep, plus you have a 520 you know and understand vs an unknown qty with no warranty.
Save your money for next rev of non-eshift when they've put to bed the teething issues.