Before revision:  This approach has partly been used by Ito et al. (2014, 2016a,b), and is proven very efficient because, unlike conventional methods, it does not require the Kalman filtering and smoothing procedures, but yields a smoothed estimate that is identical to the Kalman-smoothed estimate.
After revision:  This approach has partly been used by Ito et al. (2014, 2016a,b), and is proven to be efficient because, unlike conventional methods, it does not require the Kalman filtering and smoothing procedures, but yields a smoothed estimate that is identical to the Kalman-smoothed estimate.