Can I get stable 4K 60Hz with two external displays without MBP screen on?
Your internal screen of your MBP being on/off doesn't factor into whether the MBP can support 2 external 4K displays
According to the specifications page for the mid-2015 MacBook Pro:
Dual display and video mirroring: Simultaneously supports full native
resolution on the built-in display and up to 3840 by 2160 pixels on up
to two external displays, both at millions of colors.
This means it will support two external displays at 4K (UHD) resolution.
But at 60Hz?
According to Apple Support document Using 4K displays, 5K displays, and Ultra HD TVs with your Mac
If you use a 60Hz MST display with the MacBook Pro (Retina, 15-inch,
Mid 2015) with AMD Radeon R9 M370X graphics card or iMac (Retina 5K,
27-inch, Late 2014), only one additional Thunderbolt display is
Yes it will support it, but only one more display connected to the Thunderbolt port will be supported - you can connect two displays.
Do those monitors support MST?
The LG 27UD58-B supports DisplayPort 1.2 (MST capable) per their specifications. I haven't researched the Lenovo monitor.
What about performance with MBP15 mid-2015DG
Given that the monitors are supported, your MacBook Pro should perform as expected. Keep in mind, performance is a subjective description - what performs to expectation for one person may fall short for another.