The trade was never a good one. It's gotten even worse now that we don't even have a pick in the top ten as compensation for losing the best 3-4 DE in the NFL. This topic has been beaten to death numerous times as well.
To be honest, I thought he'd be much better in Oakland without the two-gap responsibilities, but he hasn't really lit it up. Last season was pretty mediocre. This year has been solid, but hardly a fantastic season.
Some may point to the difference in record but when you really look at it, most of the gains in the past year were on the offensive side of the ball. Consider the defensive comparisons between the 2008 Raiders without Seymour and the 2010 Raiders with Seymour.
2008: 24.2 points against per game (24th)
2010: 23.6 points against per game (20th)
Not a huge improvement. But on offense, they jumped from 16.4 ppg (29th) to 25.2 ppg (9th), which is really the key to their success this season.
2008: 4.7 yards per rush allowed and 159 yards per game allowed.
2010: 4.5 yards per rush allowed and 131 yards per game allowed.
A 20-yard difference per game seems significant, but the difference is approximately 4 carries per game fewer this season compared to 2008. When you factor that in, the run defense isn't much better at all.
2008: 32 sacks (13th), 16 INTs and 15 FFs, recovering 8.
2010: 40 sacks (4th), 8 INTs and 15 FFs, recovering 12.
Seymour has 6 of those sacks and has clearly mattered in this area, though the extra pressure hasn't led to a significant increase in turnovers.
2008: 7.2 yards per pass attempt allowed and held QBs to 79.3 rating (12th).
2010: 7.4 yards per pass attempt and held QBs to a 91.6 rating (26th).
Not a real positive change here.
So the Oakland defense isn't significantly improved with Seymour other than in sacks, and even that hasn't translated to other areas such as fewer points or pressuring QBs into more turnovers.
I know there's more to the game than just numbers, but Oakland's improvement has really been on the offensive side of the ball. Combined with the difference in schedule (2008 Raiders played the AFC East and NFC South, where each division had 3 teams with winning records vs. the 2010 Raiders playing the NFC West and weakened AFC South), and you can make the case that Seymour hasn't really made much of a difference at all for Oakland.
I still wish that pick was in the top 10 though. But he hasn't been that great since he left, and he hasn't made that team much better either.