Are these nacelles the stealthiest the J-20 will get? Is it likely to get further LO improvements when the WS-15s are deployed?
Quite unlikely any further improvements. These nacelles are custom designed for this particular WS-10 which also seems rather intended for the J-20 and designed accordingly. The nacelles are stealthy and represent the best of what the industry could offer the J-20. Highly unlikely the WS-15's nacelles can be better unless certain qualities of the WS-15 allow for better solutions if they exist at all. The only likely improvement offered by a WS-15 powered J-20 would be the obvious ones. The question is whether WS-15 would come right out with TVC and if so, how it is implemented could potentially change stealthiness e.g. if it applies a similar method to F-22 or maybe sacrifices stealthiness because some slight changes are required for the rear end to better accommodate TVC.