Changing the size would normally increase the number of bits being physically transmitted over the air for each packet; However, I don't know the behavior for BLE (I think it is limited to 23?)
I would adjust the MTU size to match the expected data characteristic packet sizes that you plan to exchange over the BLE connections. That way, the module knows that it is okay to send more data at a time if the situation occurs.
E.pratt is correct that changing the would normally increase the number of bits being physically transmitted over the air for each packet.
Thus if you want to send a data in one particular normal read / write, then MTU should be greater than or equal to the data size. Else you might have to use read/write long.
In effect, if you want to send a large packet, the speed of transmission increase as the MTU using in the connection increases.
But in general, default MTU will be 23. If you want to use an MTU > 23, you have to make sure the Client can support the same. Then Client have to send a MTU exchange request. For some Clients like mobile phones, MTU size may limited to some particular values.
You can also have a look at the following KBA :http://www.cypress.com/knowledge-base-article/maximum-packet-size-according-mtu-kba203312