IO excels at only one thing over Arsenal. 1v1'ing and 1v2'ing snipers in all gameplay: SR, dueling, regs, wPvP. Absolutely makes sniper a joke meme class, while Arsenal will struggle against a good sniper because at the end of the day. Arsenal has some of the most predictable game play.
If this is your first time playing merc or its been a while - 100% relearn it on Arsenal. I've played thousands of games as both and the REAL problem of IO is not the heat management or "casting" (which is extremely minimal in pvp, besides spamming scans while los'd LOL). IO mercs are just not scary. Sure, you can pull of a thermal double mag shot combo with dots rolling (even adding fusion and dart) but nothing compares to the chunking a priming shot>HS>stacked rail shot. 3 globals vs like 5, 6 maybe even 7? For results that are always coming out lesser. Most IO Mercs are playing to show off, not because its a good pvp spec.