Side cuts are still on the 161 and i found them ok on the 160. Felt a litlle better in releasing the water on the downwinds. But I never tested it one on one.
If you really consider a after market fin then save up some more and buy a custom. They are lighter and a better option and worth the extra bucks (although they are scarry expensive).
I have a R13 for three years now and it is still in great shape. Also the R19 will be available as a custom and be lighter then the Drake.
Remember that only that 70 cm in the water must deliver all the goods to counter the force from you and your sail against the water. It is a very significant part of the whol combo. Be prepared to invest some money in it and you will have significant greater performance.
Even if the Drake fins look and feel great (a bit heavy) I have not been using them on the 160 159 or 158. They where all sold with the board again never even been in the board or in the water. Always sailed these boards with the R13 soft from +4 upto +8. B)