The bonuses in and of themselves are minor. Adding 2 here or 3 there.
However, as an example, if you attached, say 5 +6 vit (I believe the level 4 vit is +6), to a body piece, as well as another 5 to your hands, and to your legs... and your feet... Then attach ~1 +4 per piece of jewelery, giving another 5x4 bonus, you are looking at a collective bonus to vit amounting to 140 vit bonus. Don't get me wrong, I am writing this up as an example, and do not attest that this can be done, or would be wholly beneficial. However, as an example, that would be how you would make the stats significant to have a noticeable impact.
Now the other problem with that is what the cost in time and effort would be necessary to totally stat blast your gear like that. Even though it is MUCH easier now to make a 5x meld, the impact of such a meld is significantly less, and you are still going to burn quite a bit of materia on some of the melds (e.g. ones that only have 1-2 slots as opposed to ones with 3-4 slots).