Yes. I had way more than $15000 in the account at a time. The peak "funds employed" was probably around $55K. The average was probably $30-35K so can be done on a margin account if one so desired. I didn't use CFDs.
On one hand, you could probably argue that the P/L of $13K was on the $30-35K instead, which makes the return % of ~40% far less impressive but more reasonable.
On the other hand, as I use $15K as my risk management figure, I would say the RAROC (risk adjusted return on capital) is still 78%.
I also keep a close eye on total amount at risk (i.e. entry price - current stop loss price) at any one time. And I usually manage that to be ~10% of the risk capital. I think this was a good comfort check, that even if I was holding $50K of stock and all my stops get hit at once, I am only down $1-2K in terms of capital (but also a lot of open profit).
The downside was that this had some impact on the way I move stops up - I perhaps moved stops up faster than they warranted just so that this total amount at risk looks smaller.