Is an increased MTU size possible without Data Length extension?
Yes, it should be. However increasing MTU means the softdevice needs to allocate more RAM - realistically without some hackery that means everything needs recompiling and the RAM is gone regardless of whether you use the MTU or not.
Also, what does the current Espruino stack do with an incoming gatt MTU request?
There should be negotiation performed by the BLE stack and each device should be able to query the negotiated MTU I believe. Personally I'd say the device is probably ignoring that.
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Yes, it should be. However increasing MTU means the softdevice needs to allocate more RAM - realistically without some hackery that means everything needs recompiling and the RAM is gone regardless of whether you use the MTU or not.
There should be negotiation performed by the BLE stack and each device should be able to query the negotiated MTU I believe. Personally I'd say the device is probably ignoring that.