I had a similar building than yours on my old Fire/WP scrapper (unslotted since then). I could reach something like 35% defense against most type of damage, but it was really hard to get higher without compromising other parts of the build, or getting ridiculously expensive. Anyway this toon was one of the most sturdy scrapper I ever had, his survivability was really impressive. It happened than after playing it in a few different scenarios I didnt feel the need for the soft cap anymore.
The way to get there for you would be to add a 3% glad IO proc, drop a pool (Hasten/SS) and get maneuvers, and a few more tweaks. But I wouldnt do it unless you have a very precise purpose, because I think your build is already really fine as it stands. I would even consider, for an AoE oriented scrapper as yours, to drop 2-4% defense more and get Fireball or Energy Torrent. Since I respecced my Spine/Inv with Energy Torrent, the toon is twice more fun, really I dont exaggerate. This AoE cone with knockdown is great and plays so well with Throw Spines.
Here my Fire/WP old build for info. Maybe it can give you an idea or two as it aims for the same kind of performance as yours. Good luck 
Hero Plan by Mids' Hero Designer 1,707
http://www.cohplanner.com/
Click this DataLink to open the build!
Code:
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Thanks...your build looks very solid. I may give up a few percent S&L and even drop Fitness in favor of PP & Torrent...
Here's where I'm at now...any advice/input is appreciated. I'd love to get over soft-cap without compromising power choices or other capabilities, but I understand something has to give somewhere.
Hero Plan by Mids' Hero Designer 1.707
http://www.cohplanner.com/
AFK.Farmer: Level 50 Natural Scrapper
Primary Power Set: Spines
Secondary Power Set: Willpower
Power Pool: Fitness
Power Pool: Fighting
Power Pool: Speed
Power Pool: Leaping
Ancillary Pool: Blaze Mastery