Since you asked..........
.............no, putting a Leupold on a 30-378 would not "void the warranty". I don't know HOW you could void the warranty without drilling holes in the scope or modifying it in some other way.
Leupold's are unconditionally guaranteed forever. I know of one that was driven over by a truck and another that was dropped from great height, both were replaced, no questions asked. I once offered to pay for repairs on another scope which had been misused, they said "nope, this is part of out service". I have since bought probably 15 of their scopes. An acquaintance once acquired one which had lain in a Minnesota swamp through the winter and spring, he was gonna' send it in to be "fixed" but after cleaning it up and soaking it in hot soapy water for a couple hours it checked out just fine and he put it to use instead of wasting the postage.
The statements that you see here (mainly on the centerfire BR board) regarding Leupold's "not holding up to the rigors of competition" are in the main irrelevant to one who's not shooting a rifle capable of 1/10" accuracy at 100yds, a VERY select group indeed. The "problem" is that a scope system must have moving parts for it to be usable. Asking a mass-produced item to "move".........but yet to show ZERO movement in a line projected over 100yds is a pretty tall order. Remember that the rifles are truly capable of putting 5 or 10 or even 25 rounds into a hole 1/10inch across, the ideal scope will show absolutely ZERO movement or tolerance lest it enlarge this group.
Just picture a line 300ft long and you've got to set back and make adjustments of fractions of an inch while keeping the lash in the system at ZERO....................many of the scopes used are actually epoxied solid or "locked" so that they cannot move at all and then external adjustments are added in an attempt to achieve this ZERO movement.
A look at the match reports will show you that both NF and Leupold are used extensively while some of the new super-exotics like the March are making inroads. My advice is to use the Leupold and just forget about that part of the system for a while.
al