Marlon, the 255 cams are probably a waste of time as I am sure that the standard cams already have more duration. Unless the lift and the ramp profile were significantly different, I doubt your engine would make much more torque.
You would loose top end though.
As a rule of thumb, I reckon that a cam with more than 270 degrees of duration will affect the idle to some degree.
If it is any help, I changed the cams in the 75TS for a pair of Colombo and Bariani cams which I purchased from EB Spares. They cost £320+vat. I also invested in new cam followers and valve guides at the time (seemed sensible). I polished the ports and enlarged the outer radius of the ports by 1 or 2 mm. The cams were 280 degrees inlet and 265 degrees exhaust. The inlet lift was increased from 11mm to 12mm. The exhaust was increased from 9.5mm to 10.5mm lift.
The changes did hurt the low end torque and flexibility and everything was pushed up by around 800 or 1000 rpm.
The new cams only really come on song at 4500 but really useful power is available at 4000rpm. This is no surprise as these cams do not have the variator function as there is no internal oil ways to feed the variator (ascertain if your new cams utilise the variator or not). If the cams had the variator function, I would expect better low end.
I found that the mods were not quite to my satisfaction so I got a chip from Jamie Porter specific to the camshafts, the porting and the manifolds and CSC sport exhaust. The engine no longer seemed to be out of breath at 5500rpm and pulled right the way to the (standard) 6500rpm limiter (75 propshafts do not like to turn too fast). The car still does not seem to pull much harder, but there is now the same amount of pull at around 10mph faster. There is not a slow up to hit the limiter in 3rd and likewise, 4th reaches the limiter without a struggle. In fact, the engine will probably reach the limiter in 5th (but I got spooked with 3mph or 150rpm to go!) Outright performance is now better than a 2.5 and 3 litre cars need an alert driver to stay ahead!
Worth it- yes! The only trade off is it now takes more concentration to drive smoothly in stop start traffic and is definitely an open road car. At the time it was my main from of transport over the summer months so I needed it to be tame. Your 1.8 would not need quite so much lift or duration in the cams for the same percentage increase, but would give a little more (comparatively) with the same spec. I hope you have deep pockets!