Considering that when Plax was here, there was Hines and not much else (Randle El), it's much tougher for Sweed to take away from Hines and Holmes. That said, Nate Washington was a UDFA from Tiffin who did a pretty nice job of filling a role here, had some skills and some drop issues, but at a baseline skill level I'd be inclined to say Sweed has better size, may have better hands (!), and may be better except in 40 time and vertical jump, FWIW. That's to say, in the three active years Nate was here, he averaged 568.33 yards per season, and 4.0 TDs. Best year was '08, with 631 yards and 3 TDs.
Looking at Ben's passing in the past two regular seasons, 2007 he played (mostly) healthy and had a great year, 2008 he battled the separated shoulder all year, among other dings. Still, Ben's yardage went up in 2008, even if his TD-INT and passer rating slid some. If he's playing healthy, I think we may see the TD-INT rebound some, but the yardage and other stats have been pretty close of late.
2008: 281-469 for 3301, 17 TDs-15 INT, 80.1 rating
2007: 264-404 for 3154, 32 TDs-11 INT, 104.1 rating
Of that, the WR haul (with % of total - does not include Batch or Lefty):
2008: 183 catches (65.1% of completions), 2565 yards (77.7% of total), 15 TDs (88.2% of total TD passes)
2007: 174 catches (65.9% of completions), 2385 yards (75.6% of total), 21 TDs (65.6% of total TD passes)
The number of completions and percent of total yardage is amazingly steady. The only flux, as mentioned, is TD passes, which I would imagine would rebound some, though perhaps not to 2007 levels.
Let's extrapolate this. Assuming the running game is better with a healthy Parker and a healthy Mendenhall, the passing yardage probably won't exceed the 2008 level by much, if at all. TD-INT ratio should improve if Ben's healthy, which is always an "if." So I'll throw some arbitrary numbers out... I averaged completion, attempts, and yardage, then added 5% improvement. For TD-INT, I did a plain Jane average.
2009: 286-458 for 3389, 25 TDs-13 INTs, 91.32 rating
Now, assuming about 65% of those completions go to WRs, and about 76% of the yardage, that comes to 186 receptions for 2576 yards. If you take Ward, Holmes, and Washington's % of total receptions, yardage, and receiving TDs (including Batch and Lefty), they have:
Ward 2008: 25.6% rec., 21.6% yardage, 20.6% TDs
Ward 2007: 26.7% rec., 28.9% yardage, 36.8% TDs
Holmes 2008: 18.5% rec., 27.8% yardage, 23.5% TDs
Holmes 2007: 18.2% rec., 22.8% yardage, 26.3% TDs
Washington 2008: 10.3% rec., 13.3% yardage, 14.7% TDs
Washington 2007: 13.2% rec., 17.5% yardage, 15.8% TDs
The pass distribution pattern is strikingly similar year to year. One might reasonably expect Sweed's percentage of receptions to fall in the 10.3%-13.2% range. Assuming a best-case scenario, with Ward's number slipping slightly and Sweed taking the upper range, we'll say 13%.
The yardage number bounces back and forth between Ward and Holmes from 2007 to 2008, but those two combine for 49.4% and 51.7% of total receiving yardage over those two years. One might expect a similar number, around 50%. Washington's numbers of 17.5% and 13.3% show a 15+ YPC, and one might expect Sweed to draw the same sorts of receiving routes; the infamous drops were on the kind of long passes that often went to Washington. Rather than use a % of total yardage, I'll round the YPC down to 15.0 even (giving a slight speed edge to Washington, although Sweed is stronger and may have better YAC).
Washington's TD totals were consistent, about 4 per year, about 15% of total. Sweed may offer a better RZ target, especially with teams covering Ward, Holmes, and Miller. I'll give him a bump here to perhaps 6 TDs, or 24% of the total receiving TDs.
Averaging out Holmes and Ward, and using the inputs I've described for Sweed, Ben's passing numbers (plus whatever odd yardage Batch or Dixon might add) might come to this. This assumes very little goes to MacDonald, and Sweed fits right into the Washington role; Sweed could certainly exceed and have a Plax-like year, or have MacDonald leech away some stats, but I agree with msdmnr that this may be Sweed's year to get his feet, and next year may be his year to put up big numbers. Given Arians's proclivity to distribute the ball in certain ratios, and his unwillingness to change gears suddenly, I don't see Sweed taking that second-year Plax role not through his own shortcoming, but because Arians can't let him rip.
Ward 2009: 72 rec., 839 yards, 7 TDs, 11.73 YPC
Holmes 2009: 53 rec., 961 yards, 8 TDs, 18.16 YPC
Sweed 2009: 37 rec., 558 yards, 6 TDs, 15.00 YPC
Other (MacDonald, Wallace) 2009: 24 rec., 219 yards, 1 TD, 8.99 YPC
That looks like a very safe estimate of how the breakdown might go. I'm not sure I think Holmes's YPC will be that high, but overall I think it works. MacDonald would likely get short gainers, crossing patterns etc., whereas I can see Wallace getting a couple nice deep balls, but overall I don't expect much out of the WR4 or WR5.