When Brady came into the league in 2000 your arm strength was what it was. If you came into the league with a mediocre arm then THAT's what you had for your entire career. Tom Brady in 2000 came into the league with a mediocre arem. But perhaps Brady's greatest asset was his never ending quest to improve and that was throughout his 23 year career. Improve he footwark, improve his focus, Improve his knowledge of defenses, and improve his arm. He had a guy he'd go to every off season, long before "QB coach" experts were in vogue. He also used Tom House long before House was thought of as a QB coach.
The point being Tom came into the league with a "mediocre are" and left it throwing bullets, thus ending forever the notion that "arm strength you come into the league with will be arm strength you have during your career".
Let's be clear here. A five percent improvement in Macs arm WILL make him a better and more effective passer. He has a beautiful touch on his deep passes. However sometimes his high arcing throws allow safeties to get involved. Guys with big arms can throw deep with more velocity and shallower arcs and keep safeties out of the picture. A bit added velocity makes shorter throws into tight windows more likely.
Expectations - I don't expect Mac to suddenly show up in July and be this "strong armed" QB. This is something that take YEARS to evolve into. A lot will depend on how his body evolves over the next decade. Will be be stronger and if so, will be be able to translate that extra strength into more velocity to his throws. So expectations should be tempered. I'll be satisfied if we see just some visible improvement.
Plus I'll add the refrain I've been saying for months here. Mac has the innate athletic ability to much more evasive and extend play than what we saw last season. His 4.8 40 and 7.06 3 cone is the same or slightly better as Wilson and Prescott, both considered QB's who can evade the rush and extend plays, In fact he has the same 40 time as Patrick Mahomes, though Mahomes has a slightly better 3 cone at 6.9.
This isn't to say Mac ever will be as evasive as these guys have been. But we SHOULD expect that over the next few years he will become MORE evasive that he was the year before, with the idea that in 4 or 5 years as he enters his prime getting out of the pocket when it starts to break down will become an instinctive reaction.