Keith,
Depending on your drain sizes (you need to get the water back out), you could add another 70 or even 100. That would be a good choice because if one ever failed you would have back up. It would not be the most economical solution for moving that much water, for my new large system I did a lot of pump research. I ended up with a Sequence pump moving 2500 GPH @ only 213 watts, and next to 0 heat returned to the system.
Dave,
Don't confuse flow with turnover. The tunze is great for creating lots of random flow, but you still need that turnover rate. Get that water down to the sump for all the fun activity down there: biological filtration, skimmer, additives heaters etc...
Good luck
Marc