What are the consequences of exceeding the SFP manufacturer maximum distance ?
Very simple: anything between nothing over spurious data errors (effectively frame drops) to total link loss.
The link will not come up at a lower speed - with rare exceptions (2.5/5/10 Gbit/s "smartrate" ports), Ethernet doesn't try to find the best, reliable speed. It links or it doesn't. Transmitted frames are intact or they're not.
With a somewhat reasonable error rate, the link might look like it's running at a slower rate but that's only from a higher-layer point of view where packet loss and retransmissions aren't visible.
In practice, you can often exceed the specified distance, sometimes considerably (esp. with single-mode fiber). That happens when the actual optical channel is (much) better than the specs require (not unusual), or the optical modules may have better sensitivity than the minimum. Still, you need to note that you're out of spec and technically on your own, without warranty and support.
If you do run an out-of-spec link, make sure that you have more than a single pair of transceivers that work in that place - there's nothing more 'frustrating' than having a transceiver die and finding out that new ones don't work...