HD Native may be what you are looking for and should handle your I/O needs with the current audio interfaces you have now. If you want more speed and faster opening sessions you need a faster computer(no want around it). You can check
www.everymac.com to look up your mac pro/macbook pro to campare what they score on Geekbench(64). It will give you a better idea of what you have and what you may need to buy.
You can even upgrade the CPU and RAM in your current macpro. Lots of youtube videos on it and companies that can do it for you. Go from your single quad to a six core or even up to a 12-core. Lots of options. You should also look into SSDs if you don't already have one for your main OS drive to help programs load/run faster. I know several folks who run a fast computer with one HDX card (to handle I/O, latency, etc) and than use the power of the faster computer to run native plugins.