The E34L tube can be used in any circuit spec'ed for an EL-34 as far as I know, but I wouldn't expect any increase in actual power just from using this tube. That 15% quote probably means something like that the "L" version is capable of yielding up to 15% more power than the non-"L" version in a circuit designed to do that. Even that conclusion might be open to question, but anyhow I did find that the "L" tended to outlast the regular version from JJ in a somewhat demanding amp circuit. (But I'll also add that I thought my best results of all, both sonically and durability-wise, were with the Svetlana EL-34's.)
Anyway, if these tubes are in good new shape, reasonably well-matched, non-defective, and survived shipping OK (there can never be guarantees on any of that, even if they were supposedly pre-tested, until you actually install them and they work without problem), go ahead and enjoy them, they're fine tubes and your amp will thank you for them.