I have dude, I've never needed to buffer them over a 50cm cable.
If you need specific help with tlc5940's then a few good people here can help you.
You shouldn't need an oscilloscope with the footprint you are for the MM74HC04M unless your logic levels aren't at the proper level. The datasheet says it uses 3 not gates for each gate. If you use two gates then you will get the same output just buffered.
The WS2803 looks to only need one serial data line and one serial clock line. As if it communicates through I2C. Notice on the pinout in the datasheet that its serial out and clock out are already buffered.
Page 5 of the datasheet :
2-wire Control Mode
When MS pin is connected to VCC, WS2803 operates in 2-wire control mode, at this mode, only 2 wires are needed to
transmit the grayscale data. It is a low cost solution. Only 2 ports of MCU are needed to control over thousand WS2803
chips.
I don't see the issue is seeing as how you have experience daisy chaining.