IMO there are too many variables related to how the defense defends a player. Simply put it does really matter who a player is playing with.
These are probably the best statistics you are going to get:
Bagley:
VORP: 0.1
Defensive Box Plus/Minus: -0.7 (very poor for a big man. Giles is at 0.9 which is still not great for a big man but much better than Bagley)
RPM rank: 479th
Siakam:
VORP: 3.5 (best King is WCS at 2.3)
Defensive Box Plus/Minus: 1.4 (best King is WCS at 2.2. Siakam was 2.2 last year)
RPM rank: 21st
Siakam was just a much more effective player as far as helping his team win games goes. It's really not even close right now. Again, this is just talking about this last year and has nothing to do with Bagley going forward.
Theres advanced on/off court metrics that are supposed to take account those variables you talk about.
Doesn't end here either. On/Off statistics are insane for Pascal:
On Court:
Off rating: 116.8
Def rating: 105.8
Off Court:
Off rating: 108.3
Def rating: 111.3
+13.9 net rating in 2530 minutes for Pascal this season.
There is some raw on/off the court numbers, still indicating that Siakam makes a big difference when he is on the court.
-Hyper efficient with a 62.8% TS on 20% USG. Only 12% TOV rate
-Improved himself getting to the line with a .320 FTr. Not elite, but above average.
-Spaces the floor with a consistent enough 3-ball. 36.9% on 214 attempts.
He's the significantly better player right now. Not a debate. Like we've been saying though, that can change in the next few years.
Theres some of the most valuable "box score numbers" that indicate that Siakam actually is a difference making player.
Speaking of increased workload, he's doing absolute work in the playoffs right now. Small sample of course but:
7 games:
23.3 PPG
8.0 RPG
2.7 APG
39.5% 3pt on 4 attempts,
59.5% TS
25.8% USG
6.8% TOV
.213 WS/48
5.0 BPM
0.5 VORP
His On/Off is hilarious right now too:
On court:
Off rating: 114.1
Def rating: 98.3
Off Court:
Off rating: 96.5
Def rating 96.0
+15.3 net rating with 262 minutes on the floor and 74 minutes off.
Absolute beast.
And there is some playoff numbers for the crowd that values players value in the playoffs highly (like I do).
Of course a player like Siakam is much better than a rookie Bagley. Rookies dont often contribute to winning and Bagley isnt an exception. The whole argument is just stupid, we are talking about a rookie compared to a proved great playoff level two way player that has clearly demonstrated that he contributes a lot to winning.
The real argument is wether Bagley can become a Siakam level contributor. Its not a great discussion either since they are not that similar players but I'd say that if Bagley ends up being a current Siakam level contributor, I would be relatively happy since Siakam is a player that actually makes a difference when he is on the floor. Of course Siakam level is not a "first option/build your team around" level but its fairly good level to be, a lot better than most of the players in their position. I would still hope Bagley becomes the better player of the two and thats a possibility but I wouldnt say that I'd be dissapointed if Bagley ended up being a player that would put up Siakam level advanced metrics.