All other things being equal an anti-roll bar reduces the grip on the end to which it is fitted. They transfer weight from the inside wheel to the outside wheel and although a tyre produces more grip with more weight it's non linear, so you're losing grip on the inside wheel faster than you're gaining it on the outside. They do of course reduce roll and if excessive roll is lifting the tyre tread off the road due to camber change in the suspension, then the anti-roll bar may well indeed increase grip (or in fact stop if from falling so much). The wider and lower profile the tyre the more likely this trade-off is.
The reason for the very thick anti-roll bar on swing-spring equiped Triumphs is becuase the whole point of the swing-spring is to reduce to roll resistance at the rear end, so you need to control it at the front. A more common approch in the USA is to fit a camber compensator, or Z bar. They may at a glance look similar, but while an anti-roll bar tries to make both wheels rise and fall together a camber compensator does just the oposite.
My Moss Monaco (Herald based) had a re-tempered and as I recall 'de-leafed' rear spring because it certainly didn't have a lowering block and the rear camber was pretty much neutral at static ride height. I fitted some quite soft front springs (might have been less than 100lb/in) and it never had a front anti-roll bar.
With the Triumph-Standard 145/13 remoulds it never had much grip, but the handling was fantastic. You could four wheel drift it in the dry and the steering wheel was nearly an optional control in the wet. A friend who regularly drove it nearly crashed the Moss cars demonstrator (equipped with 185/70-13s and a 1" front anti-roll bar) because it understeered so badly.
My Moss Roadster with a 120bhp Ford 1700 X-flow on MKII Vitesse (rotoflex) suspension and 165/70-13 tyres (actually all it really needed) didn't run an anti-roll bar either. It too was softly sprung and both cars had the engines further back than the Sammio, but I'll try running without the front bar and then think about fitting one only if necssary. The Mazda V6 is lighter than the Triumph 4 pot, but things are obviously different if you're using the Triumph 6 pot. If I were using it, I'd consider doing the suspension tower swap and moving it back. Thinking back to the Moss Monaco, I didn't have to touch the chassis rails, You just unbolted the gearbox mount from it's recesses and mounted it on top of the rails further back. A taller transmission tunnel wasn't a problem.
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