The Laser Interferometer Space Antenna (LISA) will enable precision studies of extreme and intermediate mass ratio inspirals (EMRIs/IMRIs), providing unique probes of astrophysical environments of galactic nuclei and strong-field gravity. Using a fully relativistic pipeline across primary masses $m_1\in [5\times {10}^4,{10}^7]M_{\odot }$ and secondary masses $m_2\in [1,{10}^4]M_{\odot }$, we map instrumental performance directly to detection horizons and parameter measurement precision. EMRIs with $m_1={10}^7{M}_{\odot }$ and $m_2\sim 1M_{\odot }$ are the most sensitive to instrument degradation, with redshift horizons at $z\sim 0.01$, while IMRIs are the least sensitive to degradation and reach redshifts $z\sim 1–3$. All prograde systems considered achieve subpercent spin precision within 3 months of observation. The full 4.5-yr mission increases the horizon of systems with $m_1={10}^7{M}_{\odot }$ and $m_2\sim 1M_{\odot }$ by a factor of $\sim 4$ and improves sky localization by 1 to 2 orders of magnitude reaching $<10{\mathrm{deg}}^2$. IMRI detection is robust against degradation, but their parameter estimation is more vulnerable due to fewer cycles in band. With the full baseline, EMRI observations constrain scalar dipole emission and Kerr quadrupole deviations below ground-based bounds by 1 to 2 orders of magnitude. We release the accompanying software and an interactive website to enable the community to rapidly quantify the scientific potential of EMRIs and IMRIs.

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• Classical black holes
• Gravitational wave detection
• Gravitational wave sources
• Gravitational waves
• Galactic nuclei & quasars
• Astronomical masses & mass distributions
• Gravitational wave detectors
• Numerical simulations in gravitation & astrophysics