What people have to remember when comparing rugby to American football, NFL players run for 30 seconds each play. Those monster you see in the NFL would be brillaint for the first 5 minutes of rugby, but the conditioning is so different, there is no way that they'd last against a quality team. One thing I think would happen if the USA invested heavily in rugby, is that there would have to be more stringent steroid tests, because it's not something that rugby has really had many problems with in comparrison to baseball, basketball and especially American Football. There is a biological reason why so many of the freaks in the NFL are so big.
If America did really want to take up rugby (and I don't think rugby has the commercial potential for US networks to really get in behind it), then you have a population of 310 million. To put that in perspective, South Africa has 49 million, France has 62 million, the UK has 62 million, Argentina has 40 million, Italy has 60 million, Australia has 22 million and New Zealand has 4 million. Adding that collectively and USA still has an additional 10 million people over all of the top 10 rugby nations. If they genuinely adopted rugby and became competitive, then they have the potential to become a force. The fact is, you couldn't chuck in a bunch of NFL players and expect them to become rugby players, you have to develop the game from the grassroots, which means highschools, universities and independent rugby clubs.