Google: "table 2-16 Light Reflection"...and google books should take you to page 217 of "CRC Handbook of Tables for Applied Engineering Science" where Table 2-16. Light Reflection and Transmission is published, (sorry not able to cut & paste...for some reason) so below are few items to compare.
Polished silver, 95% reflectance
White plastic, 78%
White gloss paint, 75%
Aluminum paint, 60%
Now as to paint sheen (flat, satin, semi-gloss, high-gloss)--
Google: "2.5 Reflectance measurements" "color for science"...and google books should take you to page 68--of the book "Color for Science, Art and Technology"...which concluded:
2.5. Reflectance measurements
2,5.1. Specular and diffuse reflectance
If a beam of light shines on a mirror at 45° to its normal, it will be entirely reflected
in the opposite direction at the same angle as shown in fig. 2.20 at (a). This mirror-like
effect is called specular or regular reflection. For all practical purposes, the light does
not enter the material from which the mirror is made but is reflected only by its front
surface.
If a beam of light shines on the flat surface of a compressed pellet of barium sulfate
powder at 45° to its normal, the beam will penetrate the surface and enter into the body of
the pellet. Particles of barium sulfate will scatter, i.e., redirect, the light in many different
directions. After being scattered many times the light will find its way out of the pellet.
The light leaving the pellet will exit at many angles from the normal as shown in fig.
2.20 at (b). This is called diffuse reflection.
Light reflected from a highly glossy white paint film will have both a specular and
a diffuse component as shown in fig. 2.20 at (c). A paint film is made up of pigment
particles held together in a resin binder. Specular reflection occurs at the resin/air interface
and constitutes about 4 percent of the light incident on the film. The remaining light
enters the film, is scattered by the titanium dioxide pigments in the paint and is diffusely
reflected. High gloss materials have very smooth surfaces and the specular component
of the reflection becomes a narrow beam.
As the gloss of a material decreases, its surface becomes rougher. The specular reflection
is a wider beam of less intensity at the specular (mirror) angle as shown in fig. 2.20
at (d). As the gloss decreases further, the specular peak widens until it disappears for a
very matte material. A very matte (flat) white paint will exhibit only diffuse reflectance.
Link to the 511 page book in pdf format (25 meg)...http://dcis.uohyd.ernet.in/~mravi/downloads/CIP/CIP-EBOOKS/color%20science%20for%20technology.pdf
All paint experts share the same opinion, higher the gloss, greater the reflection--lower the gloss, greater the absorption. Some define "absorption" and "reflection" as opposites, such that Reflection Rate = 1 - Absorption Rate (ie, item with 75% Reflection is then limited to maximum of 25% Absorption rate).
My floors, walls and ceilings are painted with high-gloss white--easy to see what's dirty and easy to maintain/repaint.
White looks nice, mylar distracts - but if you're relying on reflectivity for contributing to production in any meaningful manner you will be sorely disappointed. Our eyes notice reflected light, chloroplasts don't.
Ahhh...but remember there is an equilibrium between "reflection" and "absorption", as the sum of the two will always equal to "1", ie--if a certain paint is 70% reflective, then it is also absorbs 30%.
Another way of thinking about it is, heat and light absorption go hand in hand; hence an object painted black and placed in the sun will be warmer (reflect less light) than the same thing painted white (cooler and reflects more).
But why do some people suggest "flat white" over "high gloss white" for grow rooms? It is because light waves reflect MORE EVENLY on flat/matte surfaces...or specifically the light is more "evenly diffused" with little to zero hot-cold spots. On the other hand, light waves reflected on "high gloss" surface (same color) will always produce a greater amount of lumens, flux, par, etc because of the "hot spots"--or as referred to in the above graphs, "specular reflection". Gloss = more light but less even refection, flat = less light but more even reflection.
Every chart I've seen comparing certain reflective/absorption qualities of various materials has always concluded that a "high gloss" or "semi gloss" painted surface has superior reflection numbers than "flat paint".
BTW...I agree with Jhhnn, a review of photosynthesis 101 shows plants absorb both direct and reflected light. In fact, the exact amount of "light" which a plant leaf does not "absorb" is actually "reflected". (Sum of the two must always equal "1").