We have 2 in the family. Both have been upgraded with Aluminium hops, ZCI stainless barrels and Maple Leaf rubbers but home made nubs. Accuracy and consistency improved for relatively little cost. Parts sourced from AK2M4
CYMA 098a Platinum and others in the series are a great value entry to the would of 7.62 DMR (with a version 2.5 gearbox as standard). Haven't changed anything else internally other than to re-shim and check compression (and remove lots of excess grease). Can't see a performance/cost/benefit to upgrading anything else yet.