Well, the SFP+ specifications (SFF-8431) states that lower speed transmissions may be supported.
That “may” suggests that it’s left to the SPF+ manufacturer’s discretion. While that 2nd paragraph indicates that SFP+ cages are designed to accommodate SPFs, but not necessarily vice versa!
In any case, it may very well work in some cases, but why would you want to plug a more expensive/higher-speed capable SFP+ into an SFP cage?!? 
There’s no inherent benefit.
The discussions on other forums about various SFPs or SFP+s “sounding different” is just absolute rubbish (from overactive auditory imagination). There’s no direct relationship between the PCM/DSD data encapsulated within TCP streams over Ethernet frames, to electro-optical links. Imagine if I said the colour or size of an envelop changes the contents of the letter inside. It’s totally absurd 