Boost nades are not nearly as effective as boost core. If they were, people would be using them and a different core like shield or regen for chasing. Chasing is in the most balanced state it has been in perhaps the history of the game because of boost core. The health/time tradeoff of using boost core (21 health to get you to ~90 m/s from a standstill) is far better than a nade combo or a rocket jump.
The issue with using combo's to chase is two-fold. The biggest problem is that there is a cooldown on nade throws -- with Boost, you can immediately get to 100, then combo to get to 130-140 half a second later. Nades have a two second delay, during which time you are going much slower and the capper is getting farther away, which is often unrecoverable. Not to mention the health cost of a pair of combos puts you in territory where it takes but a few chain bullets or one patch of uneven terrain to take you out.
I like the idea of draining a large percent of energy if you grab soon after boosting, but my concern is that it won't actually solve the problem. Think of the terrain after leaving the base on the maps we play -- it is highly conducive to skiing with very little energy used. A back route boost + rj on Frost or Wintermelt takes almost no energy on the return.
I don't think what we do has to make a ton of sense -- there aren't a lot of new players to worry about, and we play this as a sport more than anything. Arbitrary rules that increase the health of the game aren't a bad thing, immersion is not a factor here.
My vote is for the moderately heavy handed approach of preventing flag grabs from flags on the stand for 5 seconds after boosting, or to slow them down after the grab is made. If it doesn't play well, we can try something else, and it should be relatively straight forward to implement.