i think you can rest easy with the TP-90 tonearm.
it is one of the best from Thorens, many think better than the Rega arms too.
i find it certainly better than the RB-300 or the entry Jelco tonearm.
the TP-90 has an effective mass of 16,5g so you will want to use a low compliance cartridge.
i would concentrate on the plinth and sub-chassis for any improvements.
look at this site: http://www.theanalogdept.com/
here you will find many improvements that you can do.
the TD 125/6 turntables are a step up in quality, and the TD 320 mkII is also one to consider.
the TD 125/6 are easier to modify for sound improvement than the 320.
the TD 160 lends itself well to improvements too.
you have the 160 now, so go ahead with that one!
replace the leads from TT to amp with a high quallity shielded lead.
use an additional support with cones between the turntable and the table top it is now resting on, and replace the thin wood bottom of the plinth with some thick solid MDF or plywood.