There's been a number of Funtana 40 conversions over the last few years. When using the 8000 mAh Thunder Power packs, duration was typically 20+ minutes per charge. With 6000 mAh packs, duration is closer to 13-18 minutes, so around 9-12 minutes with the 4200 mAh pack is just about right.
For the best overall balance of power, duration and weight, the 6000 mAh packs seem to be first choice. Typically, most use the TP 5S3P packs, or 6S3P (2 TP3S3P packs in series) for even more power. The most popular power system is the Hacker C50-10L, geared 6.7:1 on 5S3P with an 18x8 or 18x10 prop (requires taller carbon gear to clear this prop) or the same motor/gearbox with a 16x8 prop and 6S3P (can use the stock gear and has more power but more weight than the 5S3P set up).
You can see one detailed conversion here:
http://www.rcuniverse.com/forum/Hang...1258419/tm.htm