If individual instructions are faster, that means the instruction decoder isn't as good as it should be. BOUND wasn't fast because it didn't get any use, and it didn't get any use because people thought it was slow... but now that the usefulness of this instruction is apparent, it seems to me that they should make the decoder issue the necessary uops and make it at least as fast as the (bigger, more cache-wasting) sequence of simpler instructions.
Instructions like MOVS and STOS used to be slower than the equivalent series of simple instructions, but now they're much faster. Even the more obscure and far less useful BCD arithmetic instructions have been made on-par with a longer series of equivalent instructions[1] so not doing the same for the much more useful BOUND is... unusual. The amount of silicon needed to implement MPX (which includes several new instructions) seems far more than it'd take to make a BOUND decode faster.