Speaking about JPK: What about the 38FC with double keel: Will there be a big lack in speed oder stability?
No, the stability is almost for sure the same. They normally calculate the need in ballast to have an identical stability curve. The difference in speed is marginal and will only matter to competition.
If you look, somewhere in this thread is a comparative between a RM with a mono keel and another with a twin keel, made by RM yachts. The differences are small, some tenth's of a knot.
The boat will have a better mass distribution and without sails the boat will roll less, not to mention the advantage of a smaller draft and the possibility to beach the boat for repairs or anti-fouling.
There is a good reason why lots of French performance cruisers that are used mostly for cruising use twin keels. The advantages are many and if well designed the inconvenients are not significant, just a small loss in performance.
This is a good article about twin keels, a bit partial towards them, but a good one anyway:
And from France we have what I consider to be some of the most innovative and technologically advanced Twin Keel boats ever seen, although not particularly offering shoal draft capability.
The RM range of Twin keel sailing yachts... Marc Lombard, a prolific and talented designer with Open 50 and 60 ocean racing machines to his credit, has designed the most recent boats in this range....
On many large cruising yachts it is obvious that as soon as they heel, the wide body of the hull pivots the short fin keel into a position where it is presenting no lateral resistance to the water and thus the whole boat slips away to leeward.
Indeed it could be argued that Twin Keels are the obvious choice for the
modern voluminous hull shape providing excellent accommodation for today’s
sailor who still requires good pointing ability and straight tracking....
Here another slightly partial article on Twin keels.
Bray Yacht Design and Research Ltd. - The Advantages of Twin Keels
Don't let they foll you
: regarding performance the only thing better than a deep draft torpedo keel is a deep draft canting torpedo keel (even if they require foils).
But when draft becomes part of the question it is well possible that the same medium draft twin keel is more efficient than a medium draft monokeel and with smaller drafts, the advantage of the twin keel becomes bigger. At least is what the French think and they have a lot of experience on the subject.
For example, regarding the RM 10.60, the two keels proposed are monokeel with 1.95m and twinkeel with 1.65m. They say that the difference in performance between both keels is marginal, even if the monokeel has a better performance but if they used both keels with 1,65m I doubt very much the monokeel would be a match. That's their point and that's why they offer mostly and recommend twin-keels for cruising.
Performance cruisers like the JPK 38, RM or Bongo they made the boats with twin and mono keels and they will provide you with all the information regarding the advantages of both systems. All of those twin keels are high performance keels.