My guess of write speeds of 60-80% of the read speed was based on previous experience.
I obsessively run HDTune and h2bench on every flash drive, sd card, ssd and hard drive that comes into my shop. I have read and write test results for over 300 diffrent devices and nearly 100 just from MicroSD and SD cards.
Having said that, most of the memory cards I test are customer equipment, meaning they're usually the cheapest available cards since they don't care or know about the varying performance differences between higher end SD cards.
I was going to argue with your claim about how much slower write speeds are on the newest sd memory but after doing some research, you're right. In fact, higher performance NAND-flash memory has write speeds less than 20% of the stated read speeds.
In a technology overview directly from Toshiba, they compare different types of NAND flash memory and NOR memory. In general:
SLC NAND Chips have read rates about 3x their write speeds
MLC NAND Chips have read rates 7-9x faster than write speeds (and support much higher capacities)
MLC NOR Chips can read at rates of 150-200x faster than their write speeds. The read speeds aren't amazingly fast, their write speeds just suck terribly. This is why NOR-flash is rarely used for storage applications but works well for booting from flash or executing code directly from the flash memory. Think of your cell phone booting etc.