Ok here's the deal:

I drive my mostly stock 318i except for the Bilstein B12 kit (available in europe only as ar as I know) which utilizes Bilstein inverted monotube shocks and Eibach linear springs (would be interesting to know the rates of those, but bilstein doesn't answer my e-mails) and a drop similar to a stock M3.

As you imagine the car understeers like crazy and is not exactly fun to drive on the limit.

So before doing the front suspension overhaul (M3 LCAB's, Meyle Heavy Duty control arms) I thought to myself: why not change the bars while I'm at it.

So after reading many posts on this forum, I came to the conclusion that e36 favors stiffer front bar. So my idea is to stick the beefiest OEM (25.5mm) bar in front and leave the stock 15mm at the back (I don't have to reinforce rear bar mounting points with a 15mm bar right?)

So what do you think? Does the logic from race cars with super stiff springs and ~300rwhp apply to my not so stiff and close to 0 rwhp car? :)
Or should I go the classic route of super beefy at the back?

Any comments much appreciated!

It will work. If you think the balance needs more rear, You can add a stiffer rear OEM bar latter, like a 18mm, without having to reinforce the mounting points.

But yeah, start with the front.

FYI, 318tds and 316i didn't even came with a rear bar, but a front 24mm bar, so I think you'll be okay with a rear stock and a front sport package one (the 25,5mm you mentioned).

You may also want to add front camber (96+ M3 front strut hats, swapped left to right).

If you're having a problem with understeer, a stiffer front sway bar will make it more so.

If you're having a problem with understeer, a stiffer front sway bar will make it more so.
Typically, yes, but the E36/E46 tends to like a stiffer front bar (up to a certain point) to help keep the suspension more in the geometry sweet spot. But typically, stiffer reduces grip.

I agree with James.

25,5mm isn't that much extreme anyway.

You may also want to add front camber (96+ M3 front strut hats, swapped left to right).

Yeah, I'm seriouselly considering either M3 hats or camber plates...

Thanks everyone, I really appreciate how you guys help people like me do things right the first time :)