The additional stiffness of the target action and the lack of a magwell cutout is negligible to anyone but a benchrest shooter and they would have difficulty quantifying it.
I've heard it said from a lot of people that the opposite is true. The effects of barrel (and action) flex are not minimal. I know a lot of people that insist on single shot setups for most potential accuracy. Stiffness is one of the primary requirements for accuracy according to many.
I think you're way off the mark here. Stiffness is a common trait sought out by those seeking accuracy. And the easiest way to make an action stiffer is to remove the feed hole cutout by going with a single shot design. I fully expect a harsh response so this will be my last post on this subject. All I can say is pretty much everyone I've ever talked to, read or seen in videos believes what I believe - that stiffness is important to accuracy and that a single shot action contributes to better accuracy. That's true from the guys at my local club who prefer shooting a single shot rifle to the top shooters in the world.
Here are a couple of quotes the first from a
Sniper Country article on rifle tuning. They get right to the point:
"A good action has a number of important qualities. The action must be rigid."
They list stiffness as the first quality an action must have in order to make an accurate rifle. The only other quality they mention is that the bolt face must be square to the chamber. That's about as obvious as it gets. But they list stiffness above that quality. I think that shows clearly that a whole lot of people see things my way. That a rigid action is very important and we all know that a single shot action can be the most rigid.
Then there's
this quote from barrel maker Lilja. I think it states the conventional wisdom on this subject quite clearly:
"The rifle must be built around a potentially accurate action with a good trigger. ...the big single shot custom actions are the best for this application."
The Savage target action is sold as a stand alone action which could be argued makes it a custom action. At the very least it is a big single shot action which is what the people at Lilja seem to think is important.