SFP vs. SFP+

I was asked today what the difference between SFP and SFP+ is. I did really know, so I looked it up and it seems that the SFP spec provides capabilities for data rates up to 4.25Gb/s, whereas SFP+ supports up to 10Gb/s. Naturally, this made me wonder whether or not an optic that supported 10GbE always conformed to the SFP+ standard inherently, or if there are cases where a 10GbE optic might only support the SFP standard, thus having a 4.25Gb/s bottleneck.