The P4 IHS just gets in the way really, for the sake of expliation you can liken it to putting a 1mm copper shim between the core and your waterblock if you were using an exposed core CPU, (like AXP). As you can imagine in doing this you just create an extra thermal interface. People usually find good temp gains when they remove the IHS. Unfortunately some of the newer P4's are bonded to the core and will be destroyed if the IHS is removed.
Accepting the above, on the design you sketch the fins probably wont help the cooling, because they are to far from the heat source. with a waterblock, (assuming decent flow rate & pressure from the pump), the majority of the cooling effect wants to happen as near to the core as possible, and this is how the performance gains are achieved. It's a very complex subject and one I only have a passing understanding of so it's probably better to let others with greater understanding explain it in more detail. in layman's terms for an overview only, If you think about it in a logical way the further the heat from the CPU core has to spread out in the block before it is transferred to the coolant the hotter it will get at source. If you can transfer the heat to the coolant as close to the core as possible with efficiency, it will heat up less. To remove the heat efficiently as possible in a small area is not easy, and that's why is takes intricate, complex designs like Cascade to achieve it. Although it sounds as if
Brians256 has swallowed a flow dynamics book for breakfast
those are the types of things that would be considered when designing a high performing CPU block now.
As said before you are very unlikely to get Cascade or even white-water performance, but you may get near, post the detail design you have and then the experienced can point you in the right direction.
The blue toped fittings I use on the 4 outlets of the "Splitter-X" (and 6 other system blocks), are 8mm Push fits. They are designed for pneumatics (10bar rated) made by Festo. The festo type I use are plated brass, with a thin stainless sheath.
www.festo.com
There are many other makers, I also use some made by
www.Norgren.com
They work well I've had no leaks or tubes come out in use. They must however be used with the correct type of tubing, (no vinyl or silicon). I use a polyurethane transparent and it works well.