I asked this once before and got no answer. From what I can gather, the mean density of the worlds oil production is rising  and it is becoming more sulphurous. Various comments have implied that more energy absorbing cracking is needed for heavier grades to match the distribution of refinery products to market demand. Sulphur removal is also, I believe, energy absorbing. This will put a downslope on the curve of consumable products relative to the production curve.

Is anybody willing the to give a guess (or better) as to whether this effect is of significant magnitude to edge the peak of usable products a bit earlier than peak oil production?

This is an interesting twist.
Peak Oil is often discussed as a volumetric quantity.
Maybe we should be more focused on "Peak EROEI" for that determines the net work we can do
If your presumptions above are true, we must reach "Peak EROEI" before we reach Peak Oil.
Sorry, but I forgot to add:
It would be interesting to calculate EROEI at Peak Oil and then project at what point along the downslope tail that EROEI becomes one.
Perhaps we won't get a Gaussian curve afterall. Perhaps the tail will abruptly stop at some date: ER = EI.
Skylar
this not only seems right to me but it links ecology and economics. In Economics 101 the competitive firm can only operate in a lens shaped region where marginal benefit exceeds marginal cost. Due to diminishing returns the cost curve always rises more steeply than marginal revenue at some point at which there is no additional net benefit. This assumes the revenue flow is intact. In predator ecology we require energy value of food intake (ER) to exceed the hunting metabolic rate (EI), which of course gets tougher as the number of predators increases. For whatever reason the dinosaurs couldn't get enough calories to sustain themselves, though a few raccoon-like critters made it. If we don't find new fuel or learn to use less we're headed the same way.