XG-PON requires an effective dynamic bandwidth allocation (DBA) mechanism for upstream traffic to support quality of service for different classes of traffic. We propose X-GIANT, which extends GPON based GigaPON Access Network (GIANT) DBA, with validated optimisations to the originally proposed key parameters - service timers and assured vs non-assured ratio of medium priority traffic. We implement X-GIANT in a standard-compliant XG-PON module designed for the state-of-the-art ns-3 simulator, tune the above key parameters and show that mean-delay and throughput for different classes of traffic obey the XG-PON requirements and respect priorities at both light and heavy upstream loads. We also show that X-GIANT shows better mean-delay performance than Efficient Bandwidth Utilisation (EBU), a recently proposed, GIANT-derived, priority-based DBA mechanism for XG-PON, for all three classes of traffic simulated.