NO. Sorry, the TG table is pretty clear how things work. The 20% discount is specifically Economy Y/B/M to Silk Class upgrades. PE to Royal silk has its own charging regime:

http://www.thaiair.com/images/Royal_...w29_Dec_05.jpg

4800-8000 miles leg Y/B/M to Biz = 32,000 miles
other ecom = 40,000 miles (but not V/W)
PE to Biz = 28,000 miles

8000-12000 miles leg Y/B/M to Biz = 40,000 miles
other econ to Biz = 50,000 miles (but not V/W)
PE to Biz = 35,000 miles

Remember that this is per ONE-WAY sector. Also, there is max mileage purchase (30,000 for USD 600).

So, depending on the sector flown, it might be cheaper to buy a Y/B/M tix and upgrade to biz that way rather thanbuy a PE ticket