Yep, as a dealer we've done this test for each new back.
You can consult with your dealer for more thorough details and example files, but at higher ISO the blacks (especially deep blacks that have been recovered) take on a different grain structure. It's subtle, but something you should evaluate thoroughly before making a choice between them. Similar differences can be found at low ISO but only in extremely deep shadows which have a lot of recovery applied. Since the switch from IIQ-S v1 to IIQ-S v2 the differences are even harder to find in practice.
Importantly, IIQ-S is not compatible with LightRoom, while IIQ-L is compatible with LightRoom.
I just shot another wedding with an IQ250 (last wedding was with an IQ250M, this wedding was with an IQ250H) and used nothing but IIQ-S the entire day. I wouldn't have considered IIQ-L unless I was going very high in ISO.
The main benefit of IIQ-S for those who haven't tried it before, is raw file size drops fairly dramatically (processed to tiff they will still be the same size as always). My second shooter used a D800E and the raw files range in size from 39mb to 50mb. My IQ250H files range from 28mb to 44mb despite having higher resolution and more detail.
IQ250H @ ISO100 on H4X with HC50mm Mark 1 at f/5.6 at 1/800th with on camera fill flash.