Hextech Gunblade on Cassiopeia?

Not sure if this is the right board to post on... So I just thought about this. I'm not sure if this has already been discussed or if it's not in question (must build/hell no), but it occurred to me that this could be a very strong item on her. Now, not for the reasons you may be thinking (spellvamp I assume), but for the active. Cassiopeia is undoubtedly a mid to late game champ. As soon as she gets her core items of Luden's, Rabadon's, and atleast a tear, she's unstoppable (assuming that she is not incredibly behind). By full build, she can have up to 1000+ AP easily. Here's the build in question: sorc. boots, Rab, Luden, void staff, Seraph, and hextech gunblade. Runes are MP marks, armor seals, and AP glyphs and quints, and masteries are 21/9/0. Build order is tear, Luden's, sorc boots, Rab, Seraph, void staff, and hextech (most cases). I usually take cleanse and flash. At full build, 500 stacks on passive, and 1st-dragon buff, her AP maxes at 1026. The build lacks any defensive stats and relies heavily on being able to give sustained damage. The hextech gunblade compliments this it in two ways:

1. The spellvamp adds the necessary sustain to survive in the late game. As her E already heals for 16 (at max E rank I believe) +6% of her AP, you can effectively heal a static 77.56 plus the +20% spell vamped damage, which can range from 300-500 magic damage (a very rough estimate, I didn't really care enough to get an average of damage on tanks vs. carries). Gunblade is also useful for mitigating grievous wounds.
2. The active of gunblade adds a significant advantage in 1v1 and chase scenarios in that it basically offers a free E+W combo at your disposal. The damage of the gunblade active is 150+40% of AP, which equals 410 damage, and the slow is 40% for 2 seconds. The passive also E-poison combo also compliments the passive of gunblade tremendously. I put this item so late in the build because there are much better core items for the early game and building it early doesn't really take advantage of the full potential of gunblade. Feel free to give your thoughts.