In this paper, the earlier developed error floor model for peak performance estimation of OFDM transmission over multipath channel with small delay dispersion is enhanced by exploiting the model’s feature of enabling application of arbitrary cyclic prefix length, by varying the balance between the counts of advanced and delayed echoes with respect to sampling instant. Applying this by accordingly modifying the industry-standard simulator in the LTE FDD downlink environment, and adopting the performance thresholds coming out of common digital transmission standards, optimal CP length values were estimated as coming out of the compromise between the achieved performance and the so added data redundancy.