As above really.
Resources are limited but I did read somewhere that fitting a torsion bar to the Spider reduces scuttle shake and sharpens up steering responses.
Has anyone got any experiences with torsion bars?
Yes, assuming by "torsion bar" you mean to say that part which is sometimes called an "upper suspension strut bar" or something similar. It's a bar which mounts across the top of the engine bay to increase the rigidity of the front suspension. I've noticed the the term "torsion bar" used in that way on this forum. Calling a strut bar a "torsion bar" is not only misleading, it's actually incorrect. Of course you should keep in mind that I think the "hood" is something that covers the engine, a "bonnet" is found only on little girls, a "boot" is something worn by an infantrymen, and a "wing" is something found on an aircraft.
A "torsion bar", as far an an engineer is concerned, means a long straight spring which is twisted as it does it's job. The typical "anti-roll-bar", like the ones found on our Alfas, mounted transversely across the lower suspension and which connects the left suspension to the right suspension, is one such torsion bar type spring. It's used in a secondary role to control body roll and to control under-steer and over-steer rather than serve as a primary suspension spring. Torsion bar front suspensions, where the familiar leaf or coil spring is replaced entirely by a torsion bar, have been used too. The VW Beetle is the most famous example.
But I think what you're talking about here is actually a strut bar, also called a strut brace or strut tower brace.
My experience is based on experience in America with small automobiles like the VW Golf, Honda CRX, etc. modified for road racing. Typical small hatch back cars use a MacPherson strut suspension system where the spring and shock absorber are combined in the one suspension unit and the upper wishbone is done away with altogether. The entire vertical suspension load is transmitted to the top of the vehicle's strut tower including spring loads, shock loads, and all the upper suspension loads. The MacPherson strut is also the component which serves as the steering pivot. Mounting an after-market strut brace on a MacPherson suspension system often produces a noticeable improvement in handling.
Contrast this with a double wishbone suspension (like we have in our Alfas) where the spring and shock absorber (damper) share the load separately and the upper suspension loads are carried by the upper wishbone, NOT the upper damper mounting point.
A strut bar makes a great deal of sense on a MacPherson suspension. But, since the upper shock mounting points on the Alfa Brera/Spider/159/etc. carry essentially only vertical shock and spring loads, bracing them with a strut bar is much less effective.
However, any Spider can always use a bit of increased body stiffness. On our Alfas the upper wishbone mounting points are pretty close to the upper mounting points for the coil-over damper unit. In theory bracing the upper damper mounting points would indirectly help carry some loads transmitted from the nearby upper wishbone mounting points. In practice, I doubt if a strut bar is actually going to do much good, especially on a road going car.
Furthermore, because the engine bay is fully packed on our Alfas with several components blocking a direct path from one side to the other, after-market strut bars are bent to some degree. They also must be small in diameter and have mounting ends which are less than optimum. All these factors reduce the theoretical effectiveness of a strut bar.
I don't want to say a strut bar is totally useless, but I suspect that anyone claiming to detect a significant change in their car (on an Alfa Brera/Spider/159 anyway) might be simply trying to justify the cost of their recent purchase. That's human nature.
A strut bar costs roughly two thirds of what a set of stiffer springs will cost. I'd say save a little bit more money and replace the springs first. Mounting a strut bar is dead easy, but changing springs is certainly not beyond the skill-set of most enthusiasts and the only special tool you need is a spring compressor which can be bought for less than 10 Euros.