Anyways, an interesting point: various sources of haste stack multiplicatively for your cast time but it doesn't stop there. When checking your throughput you get:
new hps = (old_base_heal/old_base_time) (1 + haste1) (1+haste2)… (1+crit/2) etc..
So in terms of throughput, your sources of haste aren't just multiplicative with eachother but with all factors that increase your throughput (including SP). As they are a product, the largest product can be created by balancing the stats (not just A > B but A/B = best ratio). And this balance is quite easy to work out! Since all your throughput stats gain equally from borrowed time & enlightment, you will obtain the same ratio whether you include them or not. You can work out the balance between haste, SP, crit etc without BT and Enl.
(Unless you hit the softcap)

Maths can be fun, no? :)

I was confused probably because I tend to run around like i'm in the wild west with a pair of +505 haste on proc trinkets (Egg and Embrace).

A little niggling voice deep down the back of my brain is wondering how long before instant casts consume Borrowed Time.

Yeah, yeah, doing theorycraft off of “feeling” isn't the best way to do it, but you can definitely feel the difference when you have very little natural haste.

As for Borrowed Time, see above; my experience and the tooltip bear out BT's affecting the global cooldown.

And i think (like bob) that this BT doesnt affect GCD