The Champion doesn't deal similar damage and Champion Players can see their party member roll more dice.
Joey can see Jenny roll Colossus Slayer's bonus 1d8 damage almost every turn.
Ok so first of all it is not almost every turn, it is only if Jenny's target is damaged.
Second it is not 1d8 (4.5) more it is 1d8-2 (2.5) more because Joey has a higher strength since he has an extra ASI and he gets an extra +1 damage on each attack AND an extra 5% chance to hit on each attack. So yes if you just look at it in a base damage, without putting an AC to it and adding in criticals, Jenny is doing about 1-2 points per round more .... which is like I said
similar.
It is time to bring some actual math into this discussion though.
Assumptions:
Level 6. 75% of Jenny's attacks are against damaged foes. Jenny 18 strength, dueling. Joey 20 strength, dueling. Six four-round combats and 2 short rests for the day (24 rounds of combat total). Joey is playing a halfling to get the most out of his 19 critical range and I will give Jenny a Halfling too to increase her crit chance just to be fair. Foes have a 16 AC.
Optimized with an 18 strength Jenny is dealing base 12.5 base damage per hit (25 per round) plus 4.5 from collusus slayer. She has a +8 attack bonus making her chance to hit 65% (16.25 DPR). Her chance to land colusus slayer is 65.8% (2.96 DPR). Her chance to crit on base damage is .0525 (0.68 DPR) and her chance to crit on CS is 0.0525 (0.24 damage). That is 16.25+2.96+0.68+0.24. That is 20.21 damage
per attack action. With 24 attack actions that is
485.04 damage per day
Joey is easier to calculate. Optimized with a 20 strength Joey is dealing a base 13.5 damage per hit (27 per Round), He has a +9 attack bonus making his chance to hit 70% (18.9 DPR). His chance to crit is 10.05% (1.31 damage). That is 18.9+1.31. That is 20.21 damage
per attack action. It sounds like a dead heat, but it is not because Joey gets 27 attack actions due to 3 action surges. So Joey is doing
545.67 damage per day
Not only is joey doing more damage, he is doing it with a higher AC as well.
Joey can see Jimmy roll Superiority dice 4 times per short rest.
Jimmy rolls 12 superiority dice over the day. These do 4.5 damage extra. Assuming 24 rounds of combat and 3 action surges that comes out to 2 extra damage per attack action. Joey is doing an extra 0.7 damage for crits so Jimmy is doing 1.3 more damage than Joey on the average attack action. Again this is
similar damage like I said.
Jenny and Jimmy both have more powerful characters than Joey, but the power difference is not in the damage because they are all doing similar damage. The difference is in Jenny's spells and the control options offered by Jimmy's maneuvers.