You're still going to need a lot of manpower--2,000 miles is a HUGE distance, and there will still be a LOT of effort to breach or bypass the wall SOMEHOW. There are places where you can't see the border from 100 yards away--a 30-foot wall will not improve matters enough to justify the price tag associated with it when you consider Davis-Bacon Act "prevailing wages" (read: AFL-CIO-dictated wages). Guess what happens if Congress authorizes the wall? The AFL-CIO construction trades in the border region will all go out on strike for MUCH higher wages between the time Congress passes the legislation and the Corps of Engineers puts out the RFPs.
Another problem is that there are many places where the civil engineering equipment needed to emplace a huge wall like that simply can't go, but people can still go on foot quite easily.
Finally, you're going to have to guard the coastline and all waters adjacent to them (including the international waters at 12 miles out) very tightly, and that manpower requirement is going to cripple you. You're going to have to guard every inch of coastline between Matamoros, Texas and the Maine/Canada border. You're also going to have to build ANOTHER 30-foot high wall on the Canadian border (Canada's immigration enforcement makes our INS look like the old East German Border Guards in terms of efficiency--they just don't CARE who comes into Canada, particularly if they're just passing through to the US). And then you're going to have to mount a very tight and intrusive patrol on the Great Lakes--the 30-foot-wall idea doesn't work so well there.
You'll slow down illegal immigration for a while--and then you're going to notice huge numbers of illegals in places they've never been before.