In LTE there are many layers between L3 seen by users/subscribers and raw radio (OFDM), many more than in e.g. WiFi. Many of them (not all) have their own framing structure and flow protocols (HARQ, RLC, RRC, …) which adapt quickly to changing radio conditions. All of that is transparent to users. And all of that affects throughput and latency much more than MTU setting of 1500 VS 1480.
There are similar mechanisms in 3G (HSPA) as well.
Besides, MTU is subject to negotiation between data card and network upon connection (EPS bearer establishment in LTE or PDP context activation in 3G) and is usually set to optimum value. Therefore it is not wise to force it higher (and not necessary to set it lower) even if it works.
One can imagine that even MNOs want to have MTUs set as high as possible, nobody likes to see many fragmented packets flowing through their network equipment.
In most LTE networks (and other MNOs as well) traffic fluctuates wildly. And differences are both on hourly basis (late night VS noon VS evening) as well as on daily basis (week days VS weekend).
To verify that any change made makes things better, one should monitor and statistically evaluate performance through longer period of time, at least a week.