Basic steak on the grill

Did au poivre last weekend with sirloin, but I love to grill the meat rather than the cast iron, solved the problem by grilling, put meat on a platter covered (not sealed) with foil, fixing the sauce on the stovetop (which takes a few minutes), then dumping the accumulated juices from the meat plate into the sauce. I always thought the strength of pan sauces was that the meat was cooked in there and the juices gave it the best flavor, this way I can grill them and fix a real pan sauce too. Thanks for all the other suggestions too.
You can do it that way, but that's not really steak au poivre. The sauce created for steak au poivre derives its richness and depth of flavor by deglazing the pan of the fond left behind by cooking the steak in the pan.
I aqgree with Jim on the Mallard effect and cooking the juices out during the searing process. I like the carmalization it provides however, so I sear for about one minute per side only. The most important thing is to grill long enough to achieve the internal temp for R/M etc but not a moment longer.
You cannot cook the juices out during searing. You can cook the juices out by cooking too long whether you're at searing temps or not, but 'too long' being relative to your cooking temp.

The Maillard reaction (actually a series of reactions) happens more quickly at higher temps. The combinations of the amino acid components with the carbohydrates present combine, forming new components (many of which we identify and lump together as the aroma and flavor of searing or fond); these components continue to breakdown, recombine and reform as the process continues.

While there is a slight moisture loss when putting meat on a very hot grill or pan from an immediate evaporative effect, there is no more loss than would occur if the meat went on to a cooler grill or pan, it just happens quicker. A seared steak cooked to the same level of doneness as an unseared--or less seared--steak will have the same moisture content at the end of cooking (and lots more flavor, imo, because the Maillard reaction works more quickly at higher temps).

They key, of course, in searing is knowing when to slow the reactions by lowering the temp. If the reactions continue unabated, we get the flavor components most often identified as 'scorched' or 'burned'. A thinner steak cooked to rare, e.g., can be pretty much seared on both sides and removed from the heat entirely. A thick steak cooked rare is going to need to be seared and then moved off to a lower temp to continue cooking. Left at the high temp to cook one could certainly get it to rare, but the reactions, continuing all the while, would likely leave the steak tasting burnt.

Water activity inhibits the reactions which is why wet or marinated meat will take longer to sear than drier meat. The delay can be useful in some circumstances; dry your meat, or wipe off the marinade well, if you want searing to happen more quickly.
I should have said during the searing AND cooking process to be precise as you definitely can loose moisture at a VERY fast rate during the sear. You failed to mention the part about sealing in the juices having nothing to do with searing. The Maillard Report can be seen by doing a search for "searing" on the BBQ Forum. The very first link is a post by Stogie in reply to a "searing to sea; in juices" comment.

I assumed from your prior post that you meant " during the searing AND cooking process to be precise as you definitely can loose moisture at a VERY fast rate during the sear."

My comments were meant to elaborate on yours, mostly for others that might read this thread. I did mention the fallacy of the belief that searing locks in juices in an earlier post to this thread--page 1.