H&R sways are for non M's and their thickness is the same as the M bars.
The only thicker bars on the market are Hotchkis, but again, for non M cars. (However these bars are thicker than M bars).
Good chance these will fit in the front, but the rear is a toss up with the M diff out back.
http://www.hotchkis.net/c-808-z4-2003.aspx
Sport Sway Bars
The BMW Z4 is a unique vehicle and loads of fun to drive. Hotchkis Sport developed a Sport Sway Bar Set which enhances the enjoyment of an already great handler. We concentrated specifically on dialing-out under steer, especially through the mid-corner point of slow to medium speed corners. After testing many different wall thicknesses of our tubular sway bars, we decided on a set that is perfectly matched to offer a more aggressive feel through the corners with much improved front and rear bite. Our three position, 29mm tubular front bar along with the two position, 24mm rear offer a wide adjustability range to yield everything from slight under steer to a mild rotation without snap over steer. The set includes the front and rear tubular bars, polyurethane bushings, and front and rear bushing brackets with 90-degree zerk fittings.
Specifications
Front 28.5mm Tubular 3 Position Adjustable
Stiffness: +19%,+38%+61% (Compared to stock Z4 Sport)
Rear 24mm Tubular 2 Position Adjustable
Stiffness +59%, +86% (Compared to stock Z4 Sport with 24mm front and 19mm rear)
Item usually ships in 7-10 days.
SKU : 22817
Price: $448.95