There are a few things to take into consideration.

Size, depth and "proudness" of speaker.  Need to make sure all dimensions fit (front, back and diameter).  Some speakers with the stacked tweeter modules maybe too tall and hit/conflict with the dash cover.

Matching speaker independence to the amp.  Too low of a speaker impedance will burn up your radio's amplifier.

Efficiency of speaker.  The more efficient the speaker, the less power is required to drive it.

Power handling of speaker in conjunction with efficiency.  Optimal combination is higher power handling paired with higher efficiency.  This will allow the speaker to play louder with less power, while not being blown by applying more power.  Higher power handling with lower efficiency is less desirable and sometimes an indication of a cheap speaker.

Speaker enclosures paired with speaker design.  Some speakers work better/worse in one type of an enclosure then others.  Some are made for free air use, some are not.

Frequency range of speaker.

Some of the aftermarket companies (J&M and Biketronics) are making 7.1 speakers for motorcycles and many have been making 6.5" speakers for motorcycles for years. 

IMO, Victory should have at least provisioned for 6.5" speakers and RCA line outs on their radios with option to disable internal amps when using RCA line outs.

BTW, just picked up a Biketronics BT4180 amp for the Vision.