While bigger is better on UPS sizing, something is better than nothing and this OP has a 95W CPU and 10 GREEN HDDs which draw 5 1/2 - 6 W and only a fraction while sleeping. All together they'd draw 60W plus the CPU and mobo, I'd guess he's drawing 250W with all drives spinning... maybe a Kill-A-Watt meter would help him assess this. I looked at the curve on this 550 and he'd have 6 minutes to shutdown at 250W... this sounds reasonable to me -- I mean, how long does it take to shutdown, anyway? And would all 10 HDDs be spinning constantly? I'd ask the OP to evaluate his actual shutdown time, whether he's using any GREEN features to spindown HDDs when not needed, and whether they have to be spun UP to actually perform a "shutdown" if that's the worst case event. I'd assess these conditions before I throw in the towel -- or at least consider the cost V runtime if budget is of concern...
BTW, I have two Smart-UPSs (one 1,000 and one 1,500) for my "systems" and I have plenty of runtime... like 45 minutes or more and my worst case is my HTPC w/a 95W Quad-core, 1 SSD, 1 IDE HDD and 4 Green WD HDDs + several tuner cards... but I wanted to try to "stay alive" during brief (3-10 minute) power interruptions as well as providing CLEAN power to my Epson 8350 PJ.