No, you don't, spam is wrong, upgrading a kit with another common kit is a waste of kits, to get a lvl 5 kit (+1400 boost power), you can either use a power kit (cost 6k in base), or 3 commons (cost 3k in base), so it is twice as effective to use commons to upgrade, than it is to use PUK to upgrade other PUK.
In regards to what you said about storing kits, it is wrong see here:
If I have 1 max common kit, i have 10k boost power, if i wish to upgrade it to rare, I use 2 common items=800 boost power, then I must upgrade the rare kit with 35.8k boost power to max it, meaning to get 50k boost power (1 max rare kit), I use 46.6k boost power. So a small gain, around 7%
If i just upgrade a common PUK to max, I use around 7.6 k boost power, to get 10k boost power, which is a gain of 33%
If I have a max rare kit (50k boost power), and transform it to epic using 2 rares (3.52k boost power), and then max it (85.8k boost power), I use a total of 139.32k boost power, to get 1 max epic kit, which only gives 130k boost power, so you are losing boost power by transforming a max rare kit to epic
IfI have a max epic kit (130k), and transform it to legy using 4 epics (14k boost power), and max it (200k boost power), I get a PUK that has 300k boost power, however to get that PUK, I used 343k boost power, so I lose even more power.
So we can conclude, it is never worth it to transform kits, unless you have big inventory problems.