It's a complicated thing to quantify, which is why there is an entire industry built entirely around allocating every racehorse a rating. The Grand National is unique in that it is about half a mile longer than any other race in the fixture list, so the horses who take part - especially the ones who come back year after year - tend to be rather 'niche' performers, and in the main tend not to have the speed required to make them competitive in the Grade 1 & 2 races which are all over much shorter distances. The Gold Cup, the top-rated race for staying chasers, is run over 3m2f so over a mile shorter than the National, the second-most prestigious graded chase, the King George, is 3 miles and the Queen Mother Champion Chase, the real 'speedster' steeplechase, is over the minimum jumps distance of 2 miles.
In general, the graded race winners are rated higher, i.e. better, than the majority of National competitors, because of the way racing is organised so that more of an emphasis is on speed, or a blend of speed and stamina, than it is on pure stamina.
As a comparison, this year's Cheltenham Gold Cup winner has a current official chase rating of 165. Pineau de Re, today's GN winner, has an official chase rating of 143. Each point on the rating scale equates to a pound in weight. This means that if they met in a race, the Gold Cup winner would have to carry 22lbs more than the Grand National winner, as he is rated 22lbs better. And this year's Gold Cup winner is not considered a particularly brilliant winner compared to previous champions such as Kauto Star (rated 190 at his peak) or Denman.
So on balance, I'd agree that within the context of ratings and the hierarchy of races as both are understood within racing, Grand National winners are "good to very good" rather than "top flight". There are exceptions of course, and I'd say Long Run and Tidal Bay, who ran in today's race, were definitely 'top flight' at their peak, but are on the downgrade now.