In the paper, suitability of control quality assessment based on delay approximation for delay-free plants is investigated. To this end, the LQG control benchmark, which can be seen as a MV benchmark with bounded control variance, is compared for both linear delay-free continuous-time plants with outputs corrupted by a stochastic disturbance and their lag-delay models. Using approximated plant models, the area of achievable accuracy is then defined for control performance assessment. Control systems with discrete-time PID controllers tuned both classically and optimally in such way that disturbance characteristics are taken into account are considered and compared with the benchmark.