FirePro V5800 and the Radeon HD 5770 is that the drivers for the FirePro are optimized and certified for the major CAE and digital content creation applications like AutoCAD, Pro/ENGINEER, Houdini and Maya where accuracy and reliability are absolutely essential. The additional certification obviously takes time (and therefore money) which is why the cards are more expensive. You also have to take into account the target market. While you and I would hesitate at paying an additional £220 for what is essentially the same graphics card, most companies would not.
The FirePro V5800 is designed to be AMD's powerful mid-range performer that provides support for driving three displays, offers 2.5x the computing capability over their previous-generation mid-range ASICs, and has 1GB of GDDR5 video memory. The V5800 core is derived from the Juniper XT GPU, which is the chip found in the consumer-grade Radeon HD 5770. There are 800 stream processors to be found on this GPU, 64GB/s of memory bandwidth and its power consumption is less than 75 Watts. (Source)
The certification of AMD FirePro and AMD FireGL accelerators by software application vendors assures professional engineers, designers and animators that the AMD FirePro and AMD FireGL line delivers a stable and high performance workstation graphics environment on both Windows and Linux platforms.
The rigorous and exacting certification processes, conducted by software vendors, puts AMDs FirePro and FireGL graphics accelerators up against a battery of simulations and real-world scenarios, ensuring compatibility and stability required by workstation users. AMDs quality assurance engineers also carry out comprehensive compliance, performance and functionality verification tests, and work closely with application and workstation vendors to refine the AMD FirePro and AMD FireGL products, thereby creating the highest level of reliability and performance. (Source)
Radeon HD 5770/5870 and nVidia Quadro 4000/FX 4800 are your only options unless you get something older). Perhaps this situation will change when the computer industry stops dragging their feet and make UEFI standard on all computers? I hope so.