A Speed tie is a worthy goal if the mon you aim to tie with is likely to be relevant and you don't get a big benefit from investing in a third stat. Mega-Beedrill isn't likely to be relevant enough in the meta to forgo the ability to reliably take out common mons that would otherwise trouble Sceptile, so in this case, it is a shitty goal to aim for. The situation is very different from aiming for Speed ties on two base 110 mons because there are a lot of threatening base 110s, so you have a compelling reason to sacrifice a bit of power for a shot at beating them.
EDIT: You don't even reliably beat a healthy Mega-Beedrill if you hit first unless you're boosted. The best you can hope for is this:
252 SpA Mega-Sceptile Hidden Power Fire vs. 0 HP / 0 SpD Beedrill: 172-204 (63.4 - 75.2%) -- 6.3% chance to OHKO after Stealth Rock (guaranteed 2HKO without it)
Follow that up with a solid OHKO from one of Beedrill's STABs. Investing fully in Speed just to tie with Beedrill is pointless because you can't reliably beat it, and you potentially lose against more important threats that you can beat if you run Earthquake and some Attack investment.